Child’s iPad log in trouble

Hi folks, I am trying to set up an iPad for my child. I have created his own Apple ID and linked it to my family. The account seems to work as I can login to this account on icloud.com. However every time I try to sign in with this Apple ID on the iPad it looks like it has worked but then doesn’t. No error message just back to a prompt to sign into the iPad. I have googled, I have search these forums, but can’t figure it out. The iPad is old, but it works fine when I log in with my Apple ID. Help!