Short answer: If they’re old enough that they could make First Communion if they were baptized already, they’re old enough to make their own decision to be baptized, and go through (I know this is going to sound strange, but go with it) RCIA adapted for kids.
I know this because I’m an adult catechumen right now and my parish has a separate RCIA class going for the kids.
My parish has a seperate program for children, I help one of the two deacons in regards to RCIA. The other deacon heads what it is called, RCIC.