How can I restore Screen Time and family access to my child’s iPad?

Our kids have got an iPad each, which I have family settings for and screen time/downtime settings turned on. Son is about to have his birthday and got him an iPhone. I’ve setup his iPhone, software updates etc so it’s ready for his birthday, but since doing so, his iPad isn’t connecting to family settings. It says to make sure his iPad is connected to the internet and signed into his iCloud account, which it is, but I can’t control anything or see its location since setting up his iPhone. All devices are up to date with software updates etc. Any ideas?

[Re-Titled by Moderator]