iCloud Storage not being shared with child under 13

I have a child under 13 and in 'Settings/[My Name]/Family Sharing/iCloud Storage' it says he is 'Not Using' iCloud storage when my wife and I are (showing how many TB's in use).

I can't find any information on how to add him to use our family iCloud storage. He's set-up to use all other family sharing services without any issues (Music, Books, Apps, Arcade, TV+).

Any idea what's going on an how to add him into my 2TB storage plan?