Whether I provide your entertainment of not, the following list might be helpful in organizing your party.
Make sure to get your invitations out early and collect the RSVPs in one place.
Have film/batteries/tape for your cameras. (Yep, this list has been here a while.)
An outdoor party is a great idea and have a fallback plan for inclement weather.
A good rule of thumb for the number of guests is the child's age plus 2.
Be sure to allow time for late arrivals (there will ALWAYS be late arrivals). Your entertainer should be able to help you out in this area.
Before the party remove anything that could distract from any entertainment you are providing.
Take the time to discuss your party with your entertainer. Chances are he (or she) has been to more of these things than you have (I've been to about a thousand) and can help you craft an event that is perfectly suited to your audience.
Two hours is a great length for a party. A good order of things is to start with the entertainer. Follow that with snacks/food/cake. End by opening the presents.
Put a clear plastic sheet under the table where the kids will be eating. It will make cleanup much easier and could save your rug. Be sure to tape down the edges.
Make sure there is adult supervision at all times, especially during the entertainment.
