Hey ShawnP69,
Below we've provided the steps to prevent iTunes and App Store purchases.
1. Go to Settings and tap Screen Time.
2. Tap Content & Privacy Restrictions. If asked, enter your passcode.
3. Tap iTunes & App Store Purchases.
4. Choose a setting and set to Don't Allow.
You can also change your password settings for additional purchases from the iTunes & App Store or Book Store. Follow steps 1-3, then choose Always Require or Don't Require.
For future reference, the steps are also available here: Use parental controls on your child's iPhone, iPad, and iPod touch.
Also, if you do not want them to download in-app purchases, follow the steps in this article to disable it: Prevent in-app purchases from the App Store.
If you feel they may have access to the Screen Time passcode, you can reset and change it by following the steps in this article: If you forgot your Screen Time passcode. Once there, scroll to the section labeled "How to reset the Screen Time passcode for your child's device".
Hope the information above helps. Take care.