I would always prefer to use “they” over “it” for a child. I think me and my friends have had this conversation before - if one of them randomly uses “it” for a child, it’s likely that another one of them will pick up on it and ask if it’s the right word to use.
I thought Calypso had problems the other day trying to move all posts to another topic. If more of us try to split up threads it could get messy.