i used memmy as well and switched to Mlem over more or less what you mentioned. no complaints so far! the UI is pretty standard.

i don’t know the specifics, but i feel they’re just synonyms and they share the same age range they’d represent. in terms of subgroup, id say infant is a subgroup of child based on what i hear and say. Like i wouldn’t say i have an infant baby id just say baby. Or i don’t have a baby child, i have an infant child. 🤷‍♂️