Well, in a story like this, perhaps you're longing for the simplicity and carefree days of being a baby. You ask your mom, and she sits you down. She tells you that while she can't reverse time, she can give you a 'baby - like' experience. She could start by reading you a bedtime story like she used to, tucking you in with a warm blanket. Then she might limit your chores and let you play with toys all day for a while, just like a baby plays without a care in the world. This way, she gives you a taste of the baby life in a different way.
This kind of fanfiction could potentially be a creative exploration of different aspects of the characters' personalities. For example, if two gay characters are strong and independent in their normal forms, as babies they might be more vulnerable and reliant on others. It could also be a way to rewrite parts of the original story in a very different light. Say, if there was some sort of conflict in the original plot between the gay characters, as babies they might not even be aware of it yet, and the story could focus on how they interact in this new, innocent state.
I'm not sure if there are widely known 'gay turn into baby fanfiction' works in the mainstream. Fanfiction often exists in niche communities, so it might be difficult to point out ones that are extremely popular on a large scale. However, within certain fanfiction platforms dedicated to specific fandoms where gay relationships are a part of the story, there could be some works like this that are popular among the regular users of those platforms.
Well, one way could be by simplifying the concepts and storylines of MLP to be more suitable for a baby - themed fanfiction. For example, focusing on the cute and friendly aspects of the MLP characters, like their colorful appearances and simple friendships. Then, using a more baby - friendly language, perhaps with shorter sentences and easier words. Also, the plot could be centered around things that babies might find interesting, such as a simple adventure to find a lost toy or a friendly gathering in a magical meadow.
In the third chapter of Baby Dinosaur, Yu did not turn dark but evolved into the top predator of the dragon warriors, the Tyrannosaurus Warrior. Tyranno-Warriors had powerful attack and speed, and could easily deal with the challenges of other Dragon Warriors. However, Feather was also facing the threat of other dragon warriors. In order to protect himself and his homeland, he had to gradually evolve into a Tyranno-Warrior and display his great strength and wisdom.
If you ask your mom to turn you into a baby, she might first be really surprised. But then she could start acting like you are a baby again. She might start to carry you around, just like she did when you were little. For example, if you were in the kitchen and wanted to go to the living room, instead of you walking, she would pick you up and carry you there.
Magic could be a reason. Maybe a wizard or a witch cast a spell on the boy accidentally or on purpose. Another possibility is a strange scientific experiment gone wrong in a fictional world. For example, if some mad scientist was trying to test a new gender - changing device.