How do I reset a forgotten parent PIN code for child's iPhone settings?

Apparently when I bought my son an iPhone two years ago, it asked me to come up with a PIN code. I've not had to use this pin code ever since, because everything I've needed to do only required my Apple ID password (approving new downloads, etc.). For the life of me, I can't remember or figure out what this PIN code is, and all I need to change in the settings is the permission for airdrop so they can send photos to our iMac. Can anyone help before I get him locked out of his device?

[Re-Titled by Moderator]