I don't like my girlfriends best friend and I'm uncomfortable when they hangout
Why do you feel uncomfortable around them?
They act like children when they are together.
It is fine not to like your girlfriend's friends. It might be better if you find something else to do if it bothers you.
It's not like they're too old or ugly or anything.  I just don't like them.
That's fine. As long as you don't restrict your girlfriend and try to control her, you can have your own opinions.