Ask to buy option not in settings

I’m the organiser of family sharing for my 3 kids. I have always had ask to buy enabled. My 10 year old son just came to show me he can now download apps without my permission. I went back into settings to turn ask to buy back on and the option has disappeared from family settings. I’ve still got share purchases turned on, all my kids names are still there and I am still the organiser - there is just no option at all for ask to buy, it just doesn’t exist. Help!