How do I change parents pascode on child phone?
Child figured out my passcode on his phone. How do I change it?
Passcode for what exactly?
Why is your passcode even on his iPhone?
Do you mean perhaps the passcode for Screen time?
If so, follow the steps in the support document linked below.
click here ➜ If you forgot your Screen Time passcode - Apple Support
If you mean a different passcode, please explain which one.