Hello guguamy,
Thank you for using Apple Support Communities. It sounds like you've set up Screen Time on your child's device, but they're still able to access Find My. If so, we'd like to double check and confirm proper settings have been applied to prevent them from using that app.
First, navigate to Settings > Screen Time and select Content & Privacy Restrictions. In this menu, you'll want to select Location Services. Scroll down and select Find My, from here you can select "Never" if you wish for your child to not be able to access this feature. You can also select "Ask Next Time Or When I Share".
For more details on how Screen Time works, click here: Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support
We hope this information is helpful. Cheers.