Thankfully the solution is very simple to your problem. The only way for the Garmin to accept a setting of a password is if you also define a secure geographical spot right after the PIN code setting.

If no safe place is defined, the lock will not engage and the PIN setting attempt is void. So if it is locked it means your daughter has stored a safe location as well (you just click ok so it is easy also for a 3-year old).

At this geographical spot the GPS unit will be unlocked, and you will be able to change the PIN code to your liking or remove the lock.

So all you need to do is to recollect the specific geographical place your daughter may have played with your GPS, bring it to that place and switch it on: no PIN code is asked for when you found the correct spot. You may need to wait a few minutes for the unit to locate satellites, then it should unlock.