You cannot block an app that has been previously downloaded by a child:
Approve what kids buy with Ask to Buy - Approve what kids buy with Ask to Buy - Apple Support -
"If a child redownloads a purchase, downloads a shared purchase, installs an update, or uses a redemption code, the family organizer won’t receive a request."
If this is a purchase you have made and the item has not already been downloaded by the child you can 'Hide purchases from the App Store' --> Hide purchases from the App Store - Apple Support
See this tip on possibly being able to prevent use of this app by a child:
How can I cancel previously approved apps… - Apple Community
You can use Screen time to block the (previously approved) app. Screen time.. App Limits.. Add Limit.. Then pick app.. Set time limit to 1 minute (not sure why 0 minutes, doesn't work, but it doesn't), then set to "Block at end of Limit".