How can I reset my child's Parental PIN?

Use this link to change the Screen Time passcode on your child's device.

If you forgot your Screen Time passcode - Apple Support

Also make sure they do not know the Apple ID/Password for your device, so they cannot reset it themselves. Another setting in Screen Time you will want to shut off is the ability to change the Account Settings. This is located at Screen Time > Content & Privacy Restrictions > Account Changes.

I assume your child is not on a Family Plan then. The Organizer of the Family Plan must also have an iPhone to remotely monitor the child's activities. It works the same as the Samsung Family Plan where you could not have an iPhone on that plan either.

If your child is not part of a Family Plan that is being managed by an Organizer, then the parental controls can be managed on their device. On their device, you would go into Settings > Screen Time, then tap Change Screen Time Passcode to change it to one they do not know. When asked for the Apple ID in case the password is forgotten, do not add your child's Apple ID since that would allow them to reset it, and do not forget the Screen Time Passcode that you used.

You will still be able to change other Parental Controls on their device and make sure you also prevent Account Changes at Settings > Screen Time > Content & Privacy Restrictions > Account Changes.

Review the additional settings you can change for Parental Controls on this web page. Some of the examples explain how to do it from the parent's device, but you can also make those same changes on the child's device by going into their Screen Time Settings.

Use parental controls on your child's iPhone and iPad - Apple Support