Can't use Fortnite Crew on one XBox while another child tries to play Roblox on a different...

No problem at all. Hope that will help.

Thanks for the reply, I ended up chatting to an assistant and am awaiting a follow up as we signed out of his console and signed back in with the account with Fortnite Crew pass but it still wanted me to sign in as well so I had to send some pictures of the problem happening
Thanks for the links, I will check these now, much appreciated

Hi there! This is Ronnie, an independent advisor.

Nice to meet you and I hope you are having a great day!

I understand you need help with setting up child accounts.

This page might help you out:

https://support.xbox.com/en-US/help/family-onli...

It would be best to create accounts on each of them and make sure you add them as part of your family group so that privacy settings can be managed.

Here is another page that can help:

https://support.xbox.com/en-US/help/family-onli...

To help them use their own account and play your games, you can also set up home Xbox on your console from your account:

https://support.xbox.com/en-US/help/hardware-ne...

But just a heads up though that if your child is already using your account and already has data on it, that can not be transferred to another account as there is no option for that.

Do you know if this is happening only with fortnite crew subscription?

Are you able to test with another in-game purchase or with a completely game or store item?

Another option could be temporarily sign in fortnite on PC or a different device and purchase the subscription from there

Yes it's happened on all games I've tried to play, roblox, r6, fortnite, and apex.