how can erase restriction code in my ipad?

Your only option is to boot the iPad 2 into recovery mode, plug it into a computer such as a Mac or a Windows PC (with iTunes for Windows installed) and then you will be offered to erase it. That is the only way to get rid of the restriction since the 4-digit passcode has been forgotten.

These instructions are for if you have forgotten the screen passcode, but the process is 100% the same for a forgotten parental or screen time restriction:

https://support.apple.com/en-us/HT204306

If you have forgotten the ScreenTime passcode (previously known as the Restrictions passcode), one of the two 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, 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 (Restrictions) passcode on older versions of iOS/iPadOS, the iPad must be reset “as new”, losing all locally stored data. 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 hope this information is helpful to you in successfully resetting your forgotten passcode.