Yes, an only child is the same as any other child but typically, in my experience, PARENTS of only children tend to be a little more protective of that only child and the OP is thinking along those lines.
I am not sure if I would have let my DD go when she was 8 or not. I think it would really depend on who she went with. I can think of a few of her friends that I wouldn't have had a problem with. She is 10, almost 11 and I wouldn't think twice about letting her go now with most of her friends.
We stop parties at age 10 too but I think that if I offered ANY of my kids a chance to take a friend to Disney vs having a party, they would take the Disney route ANYTIME. Is is possible that you take a friend for your older child and then go back to Disney when your DD is 10 and she can take a friend then?