In my family and my husband’s it is unusual for a child’s friend to be at a formal function where we would all sit down at the dinner table. Even if it is Christams, it doesn’t have to be a wedding, and the plan is a dinner at the dinner table. If it is less formal, kind of buffet style not seated at one table and the person hosting is the one who has children, a couple of the kids friends might be there. If the party is at a different relatives house usually not.
Whenever I have invited my neice and nephew to a race event for my husband or I also offered to take them to one of my football games in Michigan, I have told them they can bring a friend if they want.
There isn’t a hard and fast rule is what I guess I am saying, but there are some unspoken expectations I guess of what is normal in the family. If a teen child has a girlfriend or boyfriend we would always welcome them to any event.