A four-year-old cannot type a phone number and should not have to ask permission to say hello. In Rilo they tap a photo of their grandmother, and her phone rings.
Her side is one screen. A photo of your child, and a green button when it rings. Setting her up takes one invite from you, held up on a screen if you are together or sent to her if she is a country away, and a yes from you once she has opened it.
Rilo is paid for by the family who sets it up. A grandparent is a contact, not a customer. Her copy of the app has no price in it anywhere, nothing to subscribe to and nothing to renew. The same goes for every friend, every other parent, and everyone else your child ever calls.
There is no directory and no username to look up, so nobody can find her. And nobody can find your child either. She is there because you sent her one invite and then approved her yourself, on your own phone. You can cut anyone off later from that same phone, and it stops working straight away.
Her own phone, and one app installed on it. We are told plenty of grandmothers manage that faster than their children do, so you may not be asked to help at all. No new number, no subscription, and it does not have to be the same kind of phone as yours. If you are in the same room when you set her up, she never has to open a link at all, because her phone shows a code and yours reads it.