If the account is set up, and you're seeing data but some or all Contacts are Missing, please check the following:
1. On your child's device, go to Settings-(childname)-iCloud. If Contacts is OFF, turn it ON.
Note: If you can't click (childname) because it's greyed out, go to Settings-Screen Time-Content & Privacy Restrictions and turn that OFF, but turn it back ON when you are done.
2. Go to http://icloud.com and log in with your child's info. Click on Contacts. Are the missing Contacts there? If they are not, you need to contact Apple Support, tell them the Contacts are visible on the device but you can't see them in iCloud.com. On their support page, you are looking for help with an Apple ID-iCloud, Facetime & Messages.
If Contacts is turned ON in #1 and the Contacts are all shown in #2, please contact us for assistance.