Honestly, we had the best luck with party games by going to the library and getting a book of 50s and 60s style games. None of the kids had ever played them because too many of their friends were into venue parties (Chuck E. Cheese and that stuff) or big expensive things like pony rides and reptile experiences for 30 kids. So I commend you on both a small party and simple games. The book will have a list of supplies and some simple rules.
Scavenger hunts are hard for kids who don't read what's on the list. But you can buy plastic Easter eggs on clearance, call them Dinosaur Eggs, and give each child a color (put a sticker or an index card colored with a marker of that color on a collection bucket or bag) - then have them hunt for their own color. Or just make it random and help the littlest ones so the big ones don't hog everything! What's in the egg becomes the goody bag stuff - no extra bags. The goody bag thing it out of control anyway, with parents going broke!
Make the games more fun and less competitive, and you'll be fine.
You can also do a craft, like have the kids decorate foam or wooden picture frames (using other shapes or stickers or macaroni shapes), then email a picture of the kid to the parents afterwards. You can find inexpensive supplies at the craft store or dollar store.
It makes it special for the birthday kids without too much structure. Otherwise just let them play, have cake, sing Happy Birthday, and open a few gifts so the visiting children get to see your children's happiness with the gifts selected for them. I really dislike large parties where guests just drop off gifts to be opened later. It's more like the gift is the price of admission than a true present.
