The kid with the biggest smile is the winner. An extra touch would be to take a picture of each child with an instant camera and post it on a bulletin board for the day, then send it home with them later. Bags of plastic bugs and animals are a good choice, as well as inexpensive story books. Bandannas are a fun choice too. Red ones for boys so they can pretend to be cowboys or pirates.
Pastel ones are great for girls because they become a fashion accessory.
In the housewares department you will find many fun smaller boxes and containers that could be used as treasure boxes for nature finds or other collectibles. Visiting an educational or nature store will give you even more ideas, such as a bag of gemstones. Birthday parties are special events which create memories that will last forever, both for the birthday child and the guests. I can assure you that a creative unique party with old fashioned games and door prizes will be remembered longer than an annual trek to the pizza restaurant. Plus, it will possibly elevate you to the status of The Coolest Mom in the World.