Were they purchased on your device or do they have their own? Apple provides many tools for you to manage a child's purchase but they have to be used by you. Giving a child your Passcode/Password to your phone is authorizing them to use your phone and that includes for making purchases unless you have taken action to prevent that. Only you should know your Passcode/Password. Here are the many ways to prevent a purchase:
- Require Password for all Purchases
Require a password for App Store and iTunes purchases - Apple Support
- Ask to Buy - Used to approve/deny child's purchases
Approve what kids buy with Ask to Buy - Apple Support
Use Screen Time to turn off in-app purchases on your iPhone or iPad - Apple Support
- Parental Controls - Many additional options are available to restrict access to specific apps/services.
Use parental controls on your child's iPhone and iPad - Apple Support
- Prevent downloading any app - Go to Settings > Screen Time > Content & Privacy Restrictions > iTunes & App Store Purchases > Installing Apps, and change this to Don't Allow.
All sales in the App Store are final, but in some cases Apple may provide a compassionate refund, but it should never be expected. If you are using a Family Plan with Purchase Sharing you are giving permission for other family members to make charges using your payment method, so in that case it would be essential to include at least one of these restrictions.