How to remove backups from old child's device

Hi,

I have 200 GB subscription of iCloud storage and family sharing. Now my storage limit is almost exhausted.

I've analyzed usage and I've realized, that there is one child with 35 GB usage for iPad mini tablet, which doesn't work several months and hasn;t any apple active device now, second child, has usage of 38,5 GB iCloud storage, has one active iPhone with photos and backups on iCloud and one inactive iPad with backups.

How can I delete backups of inactive devices ? It is not possible to turn them on (it is not possible to charge them) ?

Turn off of whole family sharing is not an option, becouse my wife has their photos also in my iCloud via family sharing, and she could kill me, if I ask her to change anything... :)

Thank you for your help,

Radovan

PS: I have Windows and also MacOS 11.1 devices ...