The child could be an orphan from a small, destroyed village that Naruto comes across during one of his missions. This child might have lost their family to a powerful enemy and Naruto feels a sense of responsibility to take them in.
It could be a child of one of Naruto's friends who passed away. For example, if Shikamaru died in a battle, Naruto might take in his child. Naruto would then try to pass on Shikamaru's wisdom and strategic thinking to the child, along with his own ninja skills.
Naruto would be a very loving and supportive father. He'd probably be a bit overprotective at times because of all the dangerous things he's faced. But he'd also encourage his child to be independent and follow their dreams. He'd be there to teach them ninja techniques and share his own experiences to help them grow.
It's possible that the child is an orphan Naruto found. Naruto, having known the pain of being alone as a child, decides to take the orphan under his wing. He brings the child back to Konoha to give it a home and a family within the village.
The child might be a bit of an outcast at first. Given that Naruto is from the Uzumaki clan and Tenten has her own unique skills, the child could have a combination of abilities that others don't understand. But this would make the child develop a strong and independent personality. It would also be very curious, having Naruto's sense of adventure and Tenten's thirst for knowledge about ninja weapons.
One common theme is friendship. Since they are such important characters in the Naruto series, fanfictions often explore how they could have been closer friends as children. Another theme is rivalry, showing how their competitive nature started at a young age.
Sakura could be a main character. She might be the one who discovers the abandoned child and tries to figure out what to do. Kakashi could also be involved. He has a history with Naruto and might be torn between loyalty to Naruto and doing what's right for the child. Sasuke could be another character. He might return to the village and get involved in the situation, either to help or because he has his own motives regarding Naruto's actions.
In fanfiction, such a plot is just a creative and often non - canonical idea. Naruto is canonically Minato's legitimate son. In the fanfic's context, it might be used to explore different character dynamics and add a layer of drama. But it goes against the original story's established relationships.
I recommend the following two system novels about transmigrating into children to you:
1. "Good Daddy System (Quick Wear)": This novel tells the story of the male protagonist from the future world. In order to respond to the call of the country, he participated in the examination of his parents 'work license. His goal was to save the tragic protagonist in the novel. This novel emphasized the importance of a father in the growth of a child and his influence on the child.
2. "The Fuwa are not to be trifled with at the age of three": This novel described the reincarnation of the daughter of the Emperor of Heaven, Du Anlu, who became a three-and-a-half-year-old child. The story was full of mysterious plots and the process of solving the case. It was full of fun and excitement.
I hope you will like my recommendation!
The birth time of the child of the protagonist who was madly upgrading the system was not clearly mentioned in the novel. In novels, the main characters who crazily upgraded the system would often continuously cultivate and improve their strength to continuously evolve and upgrade to a higher realm. Therefore, his child might be born during his evolution and upgrade process, but the specific birth time was not explicitly mentioned in the novel.