How can I change the amount of iCloud storage for one family member?

I've two kids under 13 set up in my Family Sharing with access to my 2TB iCloud storage. For some reason, my son has an amount set of 5GB, which is way too low for what he needs on his iPad. His sister has way more (over 200GB) and I don't understand why. I've been looking on forums to see if and how I can change this 5GB limit (as I've plenty of memory available), but I haven't found anything yet. Can anyone advise me on how I can sort this out?