I’m here to drop a probably low receptive rottmnt headcanon:
I don’t think the boys ever had a “baby” stage in the way everyone imagines. Where they would cry and need care in the same way as a human baby.
I think they were closer to turtle babies- as in able to move and care for themselves from the get go, since turtles are rarely cared for by a caretaker. They probably managed to find themselves food and could eat solid foods from their very first day as mutants. Splinter likely just made sure they could reach it and had access to it. He could probably sprinkle pizza bites on the floor and they would find and eat them.
They probably learned to walk bipedally when they were still very small, and run before Mikey was even one. And they likely didn’t struggle for as long as humans babies do, with stumbling and falling over.
Their development was likely fast tracked, causing them to speak early, whether in chirps or English. Their brain functions that are unique to humans, such as emotion cognition and thoughts of ID and social hierarchy, would have been what took the longest to develop, and what Splinter would’ve had to teach and explain the most.
But in terms of just basic survival, the boys probably took care of themselves. Not necessarily because they were neglected, but because they could.
