No birthday or campout would be complete without food. Since you'll be outdoors, plan on traditional outdoor fare like hamburgers, hot dogs and potato chips. This type of food is relatively inexpensive and easily acquired. Brush up on your grilling skills and be sure there's plenty of charcoal or gas for the event.
Remember, not all diets are alike. Especially when feeding children, be aware of any food allergies and special dietary needs they may have. As the host, you're also the chaperone, so be sure that any special meals are given to the proper kids.
Keep it simple. Your own yard would be ideal, but this may not be possible. If you know someone with a larger yard, ask to use it. Whatever location you pick, make sure it is safe and convenient. Campgrounds are another option, but they can be remote and often provide only rudimentary facilities. If an emergency arises, or a child begs for home, be able to provide a quick solution.
Once you've established a location, thoroughly review the environment. Rid it of all potential hazards. Remember, once nighttime arrives, light will be relatively limited. Children will be playing, and the last thing you need is a child injuring himself by tripping over the sprinkler head poking out of the ground.
This could be your largest investment. Again, you're the host, so you may be providing the housing and the sleeping bags. Don't be afraid to ask other parents to supply their children with these things, if they already have them. If they don't, you can rent camping equipment from camping/hiking stores or general equipment rental stores. If you do need to make purchases, check out your local army/navy surplus store. Equipment will cost much less there. In any case, look for spacious tents, and be sure to supply enough of them. No one likes to be crammed in with too many people in too small a space.
Arrange your campsite intimately. Children find safety in numbers, and not all of them will be comfortable with the idea of sleeping outside, away from home for an evening.
Keep your campers entertained. Left without televisions, computers and video games, children may grow restless and bored. Campfire story time is always a favorite. Polish up on a handful of stories--the website ultimatecampsource.com offers a wide selection. Add your own personal twist to these stories, and keep the stories light and funny. Horror stories, while sometimes entertaining, will not help kids sleep well at night.