Right after the mission to Iwa, Rei and Konan filled in paperwork for a long break which didn't end even after a whole year. Apparently, long breaks were not only possible but even encouraged. Being a ninja was physically and mentally demanding, plus while the payroll wasn't extraordinarily high, it was enough to save up for a few months of not having to go to missions. People did take long 'training' breaks, 'recuperation' breaks, and so on... The village system was also accustomed to this, having a flexible call system. Being on break didn't really mean not being called upon in an emergency, special request, or a mission your team was best for. In Rei's and Konan's case, it was the demolition missions. During the year, they were out of the village for a week two times too when there was nobody specialized in the requested mission but other than that, they focused on training Mei.
After their first 'outing' on the request of some noble from the Land of Noodles... again, they even managed to bring Tsunade presents! As they were coming back, they encountered a village raid by the Kaguya clan which was quite an opportunity for them since they still needed to get one Kaguya specimen for Tsunade. These raids were not actually suppressed by Kirigakure at all. The Kiri shinobi were told to 'leave them be'. The Kaguya knew their limits and didn't push it so the raids were mostly once per year. But even then, losing one village per year must have been irritating at the very least for the governors of the Land of Water. And yet, nothing is done about it. Because they were a clan that migrated from faraway land to the Land of Water during the first shinobi war. Because of their 'loyalty' to Kirigakure. The loyalty that everybody knew meant shit as nobody EVER seen a Kaguya clansman in Kirigakure. But it was a signed agreement and going back on those by the Mizukage by attacking them would be alarming to other clans.
But Rei and Konan had no such worries. Let the Mizukage deal with the fallout, they thought as they rained death upon the Kaguyas who were happily massacring and raping the defenseless villagers. The 'specimen' they 'politely' asked to come with them, was a guy around fifty who they found forcing himself on a small girl. Rei instantly thought... 'Hey, if someone deserves the tortures of Tsunade, wouldn't it be this guy?' And so, the completely voluntary participant of human experimentation was chosen.
When the people in Rei's dimension needed to realize how good they have it, they often went to hear the music of the guy as Tsunade was 'playing' with him by researching his contents. It was mostly Mei though as she was having quite a bad time by being trained to the bone by Konan and most importantly, Rei.
Rei decided that it is important to make Mei prepared no matter how young she was. If Itachi could do it, so can she. While he wasn't too harsh on her... that was Konan... he didn't let her slack off and came up with various games to keep her entertained during the training.
They played hide and seek, the ninja-style. Rei hid and Mei had to find him thanks to the prepared tracks he left behind. Once she found him, she had to chase him by any and all means necessary which more often than not meant running vertically on trees or chasing and being chased on the water surface that made Mei quite competent when it came to chakra control.
Next Rei was playing catch with Mei. He ordered her to make an orb of lava and made her keep it hot for the entire game. During it, as he was catching it with his hands covered in suiton chakra to prevent himself from getting burnt, he was also injecting a minute amount of his own chakra into it which made it harder for Mei to keep it conjured. Considering the abrupt changes in the orb she had to control, at first, the ball dissolved more often than not. But at least it was getting more fun when she actually learned how to do it. Without even noticing, Mei learned how to control her Lava Release to quite a high degree.
Rei obviously made her do much more and mostly veiled it in the disguise of games but these weren't as important nor did they do them regularly. The last thing he had her doing was more of a chore. Mei was in the duty of providing steam for the hot springs and keeping the water warm with her boil release. Simultaneously at that.
Konan, Tsunade, and Mei were in the hot springs after a hard day of training, soaking up in the water. This was their daily routine. The girls bathed with Mei to keep her company and create a sisterly bond between themselves but it was also to make sure she was not slacking off during her chor-, ahem, training. Well, most of the time. Right now, Mei was sitting in Tsunade's lap as she slept, her face buried in the woman's bust.
"She seems somewhat tired." Tsunade worriedly said as she put the palm of her hand on the girl's forehead but then realized how foolish her statement sounded as she saw the deadpan look of Konan. "Well, more than usual, I mean." She awkwardly added.
During their time together, Konan could see that Tsunade created a deeper bond with the little redhead. At times, she even suspected the blonde is channeling her regrets by showing incredible sisterly affection for her but then came the day of Tsunade's training and all suspicions of Konan are blown away due to Mei coming half-dead out of it. Tsunade was... brutal, to say the least. At least she had the common courtesy to heal any and all broken bones afterward.
"Obviously," Konan said and smiled slightly. Tsunade was utterly unsure if the smile was due to the topic or the newly finished origami that was just deposited on the water surface though. "I've been putting her through my territory training for seven hours straight." She said, making Tsunade wince.
"Did she at least reach earth papers?" She asked, protectively hugging Mei's sleeping form closer. Konan's territory training... the one with three territories of water, earth, and outer territory of wind papers. Not to say that even the outer territory was brutal but to run her through it for seven hours straight...
"Nope." Konan popped the 'p'. "She barely reached the middle of the wind zone but even that's already commendable for her age." She distractedly continued but when she saw Tsunade's reproaching look, she frowned. "What... don't give me that look! You are by far the most brutal out of the three of us." She said lazily, making Tsunade lower her head in embarrassment.
"I do it for her own good." She muttered, her fingers distractedly playing with Mei's hair.
"Sorry for springing up on you like that." Konan sighed tiredly. "It was a long day for me too. The girl just doesn't quit." Konan smiled at the oblivious Mei but then she sniffed. "I raised her so well!" Both Tsunade and Konan started laughing at that. They were aware that Konan was guiding the girl to the personality she wanted her to have. While Tsunade felt she should be put off by such a thing, Konan's arguments were just too on point for her to disagree. Cue, the will of fire. That was enough to shut her up for good.
Despite Konan's attempt to look cheerful, it was obvious to Tsunade the woman was exhausted. While Mei was trying to breach the territory of wind, Konan had to constantly supply chakra to all three of her territories to keep up appearances. It must have been incredibly chakra draining. Tsunade smirked and nudged Konan. "Don't worry. Later we will take a bath with Rei... I am sure that will 'wake' you up." She wiggled her eyebrows at her, making Konan bump her shoulder into hers.
"Ah... you pervert." Konan quietly commented while blushing slightly. This was their routine too. When Mei fell asleep, it was time for their fun time that more often than not happened in the hot springs.
"And proud of it!" Tsunade smirked and raised her chin high to show her feelings about the matter but suddenly froze when her peripheral vision caught two green orbs intently peering at her with a curious gaze, making her go rigid. "Ah? M-m-m-m-mei!?" Tsunade shrieked.
The little girl just narrowed her gaze at Tsunade. "Tsu-chan, are you a pervert?" She asked, finishing Tsunade off. Mei then turned to Konan, her face sporting a huffy pout as she asked her. "Why aren't you letting me join your fun with Rei?" She asked in true and innocent curiosity and Konan had no answer for that kind of question. She was just opening and closing her mouth, completely speechless.
After a while, Konan recollected herself while inwardly cursing Tsunade's big mouth. Her face now sported a bit broken smile as she said. "Mei-chan, it is quite late. You should go to sleep. Tomorrow you have training with Tsu." She told the girl who wasn't really bluffed into changing the topic but was instantly scooped up from Tsunade's lap by Konan's clone that proceeded to carry the flailing and protesting girl out of the hot spring while Konan chuckled. "Konan: 1, Mei: 0." Konan whispered in amusement, only to hear Tsunade's snickers.
It was only later that day, both Konan and Tsunade were groaning, huffing, and puffing from irritation as Mei decided she would not go to sleep and join the 'fun'. She might not have known what Konan and Tsunade meant but the word fun, she knew. And she wanted to have fun. Rei only chuckled as he bathed alone for the first time in a while since both Konan and Tsunade had to show Mei what they meant by 'fun', or the girl gave an ultimatum that she won't go to sleep.
"Ah~, such peace..." He leaned his head back, half-asleep in the hot springs while enjoying the irritated sounds of his women he heard through his spy seal. Who said one needed a theater to enjoy a good show?