While you try your best to make sure everything is organized beforehand, this is actually becoming a common trend at birthday parties. Sometimes, people put the event on their calendar, but they forget to RSVP. Sometimes, one of the party guests has a brother or sister who really wants to stay. Then of course, sometimes a parent may want to take advantage of the opportunity to have a kid free afternoon, and he/she will ask if their other child/children can stay. To prevent frustration, I always tell people to have extra on hand or make it known beforehand that you need a solid count. If you are throwing the party yourself, you can purchase extra supplies, and if you do not use them, you can return them. If you are working with a party planner, like myself, he or she will usually bring extras along. If you are in doubt, please ask.
Now sometimes this might not work. If, for example, you are having a stuff-a-plush party, spa party, or crafty type party where you have to order the kits in advance, you will have to decide beforehand what to do if someone extra comes. If you do not mind having an extra guest or two, but you do not have extra favors or crafts, simply say, “Sure, he/she can stay, but I only have enough for those on the guest list (or RSVP’d). ” If you do not want or cannot have anymore guests, then be polite, but firm and say something like: “I would love to have your child stay, but I do not have enough favors to go around and do not want your child’s feelings hurt.” By saying it like this, you are recognizing the potential for hurt feelings.
When in doubt, use your best judgment, but remember to always be kind and firm. It is your child’s party after all, and therefore the ball is always in your court!