iMessage activation issue on iPad in family account

We are experiencing issues with iMessage on my daughters iPad. The Apple account in question is in a family account. In settings the Apple account tab is faded out as is the iCloud tab and we cannot access messages/iMessage.

When we try to select messages, we get a pop up saying ‘sign in to your Apple account to activate iMessage’ however it won’t let us input the password. The iPad doesn’t have a SIM card and is only used at home on the home WiFi. It has been updated to the latest software, network settings have been reset, it has been switched on and off several times, time and date settings seem to be correct. We are lost and with it being summer holidays in the UK she is using the device to communicate with friends. Can anybody help resolve this issue?

[Re-Titled by Moderator]

Original Title: iMessage Activation