how can i control what my kids buy if they have an adult account or are a teenager

Hello ~ How old are the young people? This is an excerpt from the article you came from.

*Who can use Ask to Buy

Families can use Ask to Buy after they set up Family Sharing. The family organizer can turn on Ask to Buy for any family member who isn't an adult. It's on by default for any children under 13. You'll be asked to set up Ask to Buy when you invite anyone under 18 to your family group.1

If a family member turns 18 and the family organizer turns off Ask to Buy, the family organizer can't turn it on again.

~Katana-San~

Hello ~ I sounds like you can use “Ask to Buy” Have you tried that?

~Katana-San~

Hello ~ It should be on by default for under 13 .Explain exact what is happening.

~Katana-San~