Obviously, your child is sweet and perfect in every way, but the same can't always be said for his or her guests. Sometimes a guest shows up and for whatever reason, can't just seem to get with the program. Maybe he's tired, maybe he's nervous, or maybe he's just plain rude - whatever the case, you can't let him hijack the party. The first thing to keep in mind is that the child is just that, a child. She isn't that one college roommate that you had that ate all your food. She isn't your boss demanding that you turn things in yesterday. She isn't your bossy mother-in-law, she's a child who may or may not know that she's behaving badly.
Having hired entertainment can be a big help with difficult guests. Because the party tends to be focused and busy, entertainment can help prevent difficult children before they get started. Because these entertainers have a lot of experience dealing with unruly crowds, they also have great tips at hand for keeping things under control. If you need to talk to a difficult guest, be calm and simply state the problem. "Johnny, we aren't having cake yet, please move away from the table." If the problem continues, you may need to pull Johnny away for a gentle conversation. Try and find out what's getting to the child, is he bored? Hungry? Is it possible that other kids aren't being nice to him? See if the child can be given a special responsibility, or if he's feeling overwhelmed, a quiet place and activity for a while.
If you start to notice that a child is going to be an ongoing problem, consider assigning another adult to handle the situation so that you can give your birthday boy and the party the attention they deserve.