iPad and iPhone both running iOS 12.4

I have set up family sharing and I'm the organizer. I have tried to set up screen time on my device (for my child) to sync on my child's device. However, it's not syncing, what should I do? As I've said, we are both using the same firmware. I have backed my iPad to icloud, and still not syncing. He has an iPhone 5, but I'm assuming if we are on the same iOS, there shouldn't be a problem? I can't see anywhere on Apple's support website, any issues with incompatibility.