iCloud + subscriptions

If you have invited them to a Family Group, their own plans can be replaced by the Family One in iCloud settings on their devices.

You should not need to actually cancel them, and if you do cancel them before the Family storage option is activated it can cause the deletion of data exceeding 5GB form their iCloud accounts. Do not manually cancel their storage.

See here for how to move them all to the Family Storage safely: link-> Share iCloud+ with your family - Apple Support

Settings > Your Name (at the top) > iCloud > Manage Storage > Change Storage Plan > Downgrade Options.