I don't understand the sign-up process - who am I creating an account for - me or my kids?

When you first create an account in this system, you will be adding in yourself first. That makes you the primary or main account holder and once that is done you can then add in your family members so that they are attached to your account. That way you can register all your kids (and yourself in the adult class) at the same time without having to login to separate accounts for each child. It keeps your history and receipts together as well under that main account for future reference.