Accidental parental lock on iPhone

I assume my sister gave my nephew (2yo) her phone to play games and keep him distracted while on the drive back home. He put a parental control passcode on her phone. How is that possible? I don't know. But now she can't access her phone or even get out of the app because it requires a six-digit passcode.

What steps do we need to take? I have no ideas at all.

[Re-Titled by Moderator]

Original Title: My nephew put a Parental Control code on my sister's phone, and she doesn't know the password.