iCloud storage

So, when you add your kid to your iCloud+ plan via family sharing, it will count towards your capacity, but their files and photos etc. will be kept separate from yours. You should get as much storage as you need to cover your family.

Create an Apple ID for your child – Apple Support (UK)

Family Sharing - Apple (UK)

Probably because you signed into your account, instead of setting up his own account. You aren’t supposed to share Apple IDs. Did you take a look at the links from my previous post?