What happens when you try to login, and you are trying to log in from scratch (i.e. entering the id and password), it's not still logged int from before changing it (What to do after you change your Apple ID or password - Apple Support) ?
Are you following the steps in the Apple Support article (link below) that King_Penguin posted earlier?
Question: i’ve reset my childs appleid password but then cant login with it. ive repeated the process now 5 times with no luck. i can login with my appleid no problem