Does my bank need to be aware of my child first?
Do I need to, or should I, add anything to the back of the check where I would normally endorse it?
If it is not a large amount, it may not be worth it to you to open a new bank account for your child at this time.
In that case, your bank should let you deposit it into your own account (you can keep track of the fact that your child owns it yourself) or cash it.
In times that I have done this in the past, I have endorsed the check like this:
My Name, father of Child Name
Have the child open an account. Deposit the check to that account. Good introduction to the concept of saving, and needs to be done eventually.
If they are too young for this, opening a custodial account in their name might do the job. Ask the bank; I'm sure they deal with this often.