How to take a child off my account

Sign out of your account and then sign in with their Apple ID. You might want to consider Family Sharing.

Apple ID – Create

Family Sharing and Apple ID for your child

Family Sharing

Family Sharing -What types of content can I share?

Family Sharing - If you don‘t see your family‘s shared content

Family Sharing - purchases and payments

Family Sharing – share iCloud Storage

Family Sharing - Share an iCloud storage plan with your family

Family Sharing and location

Presumably your child has her/his own Apple ID, and you have your own. So you're asking how to remove the child from Family Sharing.

Leave Family Sharing - Apple Support