Because 18+ parent can legally bound to laws of Infinityfree and they can support their child to follow the rules…
This can be even good for learning
Welcome to the forum.
The T&C’s are quite clear on this:
A child under the age of 18 cannot have their own account.
But that said, if you, as a parent had an account, and your child then built a site, and you hosted it here for them, to help them learn… In my opinion that would be ok. But I’m not the owner so you’d have to wait on admin to be certain.