How do I separate devices?

Note Apple ID should not be shared.

It identifies one person (ID). Each kid should've their own Apple ID.

This is what you should do with the iPad(s) before you gave them away,

read here for instructions => What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

To share books, apps and games purchases, setup Family sharing, read here => Family Sharing and Apple ID for your child - Apple Support

If 13 years old or older.

Apple ID - create a new.      

Apple ID – Create         

If under 13 years old, you will need to set up Family Sharing to create an ID.

Family Sharing and Apple ID for your child.      

Family Sharing.         

Family Sharing - Set up.