God parents legal or more to it?

In order for someone to be God parents.. What is the way of doing that? My son thinks just cause you say.. So and so(his friends) are his God parents because it was asked and they excepted. Isn't that something that has to be legal like in the parents will? Maybe I'm wrong?. Doesn't her parents and his parents(us) have rights first if anything was to happen to the both of them later in life?. I just don't think verbally saying so makes them "legally God parents".