iCloud 'not using' status for family member in Family Sharing

I have family sharing for iCloud but one of my family is showing as ‘not using’. I don’t seem to be able to choose on that name because it says “not using”. How can I change that on his phone or is it something on his phone? He is the organizer but I’m the one who purchased the iCloud and it’s available for sharing but not too him. Just my two kids.

[Re-Titled by Moderator]

Original Title: iCloud “not using” mean