iOS Screen Time Help!?

[mac98aop]

Hello all

I'll describe what's going on in the hope someone can help?!

One of my children handed down their iPhone 5S when they got a 7 for their birthday.

The 7 is all setup and working.

I factory reset the 5S but...

1. It routinely, regularly (multiple times per day) cancels all the Screen Time settings.

2. It has become confused with who is using which device: when I enter Screen Time, there is an error message assigned to my daughter's account implying she's still using the 5S (albeit her Screen Time is working fine on her iPhone 7.)

I assume the two are linked?!

I've Factory Reset the 5S, but it hasn't helped.

I don't really want to log both out of iCloud and start again, but fear I might have to.

Never had this before and we've 6 accounts on Family Sharing etc and Screen Time.

Any help sincerely appreciated.

Thanks

Adam