ipad keep asking password/icloud although we have key in to unblock it

Dear Apple Support,

Please help me.I bought ipad for each of my kids (5) of them . 2 of my kids ipad being blocked and it says that this poor kids change their icloud id / password. they are just 7 and 9 years old and all the setting done by me and i write and keep all the setting id but still fail to do so.

Please help because i believe this can be solve and my kids love APPLE brand so much as it blend with them.

Sad MoM,😟

Rina Shahar