It is not uncommon for the child to forget their code. Thankfully there's a quick fix to recover their PIN number!
Go to your parent app, Swipe up, click three dots in top right corner, choose REQUEST UNLOCK CODE. A 4-digit code is now generated.
Next enter this 4-digit code on the child’s lock screen and it will now unlock.
This will also reset your child's pin and he/she will have to create a new one.