Parents create the main account first. From there, you can add one or more child profiles by tapping the + icon next to your family name.
If your child already has an account, you will receive a join family code. All your child needs to do is open YOMU, tap their profile and select Join Family. Then then can enter the join family code.
If your child does not have an account, you need to fill in their details before getting the code to share with them.