Keep in mind you do not have to be an account administrator(manager) in order to make a payment. ANYONE can make a payment. Account administrators(managers) have the authority to make other purchases/changes on your account which you would be liable for.
As long as your granddaughter sets up her own MyVerizon account with her phone number simply as an account member she would be able to make a payment.
As far as I know, she would even be able to make a payment via her handset without even setting up a MyVerizon account. Although I have never made a payment in that method, I believe it is an option.