3 Xbox Consoles - 1 Family and they can't be played at the same time

I have just bought my daughter an Xbox One - S for xmas.

I myself have an Xbox X which I also recently bought and my other daughter (A) already has my old console.

Daughter A - Logs her Xbox on using my account as per above.

Daughter B - Logs her Xbox on using her account using a secondary email of mine (so I can manage it) as she's under 18.

Dad - His Xbox is turned off.

Daughter A logs on to Fortnite on her xbox

Daughter B logs on to Roblox on her xbox

They respectively get a message saying you can't both play at the same time.

Why is this? How do i stop it from happening and they both keep all their achievements etc to date. Seems to me only way is to delete profiles, start from scratch and pay for the games each themselves because nothing i've tried has worked.

I've added them both as family members to my profile. Nothing changed.

I removed and re-added their devices to my account. Nothing changed.

I set each (independently) of the 3 consoles as "Home" and nothing changed.

Can someone please help?

:)