please help me to reset screen time passcode on my child ipad

If you have forgotten the ScreenTime passcode, the procedures outlined within this link should resolve the problem:

If you forgot your Screen Time passcode - Apple Support

If your iPad is capable of supporting iPadOS 13.x or later, you should update to the most current version before attempting to resolve a forgotten ScreenTime passcode. The most recent iPadOS updates have introduced a new mechanism whereby a forgotten ScreenTime passcode can be reset without wiping the iPad.

To reset a forgotten ScreenTime passcode on older versions of iOS/iPadOS, the iPad must be complete reset “as new”, losing all locally stored data:

Settings > General > Reset > Erase All Content & Settings

In this situation, you’ll not be able to restore your data from an iCloud or iTunes backup - as the forgotten passcode is integral to the backup.

I had this same problem for my son's ipad (there was no "forgot passcode" prompt when I tried to enter or change the ScreenTime passcode). In the end, I had to set up the ScreenTime Passcode from the Family Sharing menu on my own iphone instead of from my son's ipad.

I had added my son's ipad to my Family Sharing group, but I had not yet set up the ScreenTime passcode. From my iphone, I went to Settings>Family>(my child's username)>Screen Time. Then, the app asked me whether I wanted to set up several parental controls (similar to the steps described in Set up Screen Time on iPhone - Apple Support). At the end of all that, I was prompted to add a 4-number Parental Controls Lock. Now I can successfully enter that code into my son's tablet to control the Screen Time options (or I can control similar options from my own phone). I hope this works for you too!