I love any occasion to have fun and dress up! Birthday parties for kids are fun because they're so imaginitive and excited to put on costumes and play a part. There are so many themes that are possible with a co-ed party, but I'll just suggest three to keep this shorter.
Since the party seems to consist mainly of girls, I'd say the best fit would be a castle party. Of course, many girls love to dress up as princesses but your son and other boys can be princes or even knights. You could have a royal feast, pick up some horses (I saw some of those horse toys on sticks at our local dollar store) and make an obstacle course that they have to navigate while pretending to ride the horse, decorate crowns/tiaras, play musical chairs and each kid wins their own sceptre, and you could even draw a big castle on a roll of paper and pin it up for kids to color during the party.
Another cute co-ed theme is a Rodeo party. The kids dress up as cowboys/cowgirls. If you find those horses at the dollar store you can still create a rodeo-themed obstacle course using the horse. Make up some games using the horses. For example, have them place the horse on the ground, put their forehead on the horses' forehead and spin around the horse three times, then have the kids stand and walk to a designated finish point (make sure there's nothing to hit a head on because they'll be dizzy and uncoordinated). Find some white or solid bandanas and have the kids decorate them. Play horseshoes.
My favorite home party that I've repeated is the Circus/Carnival theme. Kids can dress up as circus animals or clowns. If you don't have kids dress up for the party, you can offer face painting instead (if you or a friend can do it). Kids can play games to win tickets (found at WalMart and some party stores) which they redeem at the end of the party to select their own party favors. You can have any combination of games at the party (i.e., bowling, throwing a ping pong ball over a collection of plastic cups to see which it lands in, etc.), the kids don't care what the actual games are -- they really just want to win the tickets. Offer a handful of, say, ten games and ask parents to man each station as well as give out tickets. The kids can go to each station in any order. We've also done a fortune teller and fishing for treats. After the games, the kids can line up, someone counts their tickets (I told all the parents to be leisure with the tickets since they come in such a large roll. The actual ticket count doesn' t matter. I always just pretended to count and when I'd announce the ticket number, the kids would all have similar amounts). Then have a line of a few parents and send the kids through one after the other. Tell each kid they can choose X # of items (whatever you choose, depending on how much you bought, everyone gets pretty much the same amount).
As for food ideas, I think finger foods are easy for the kids and you. I have been successful with pb & j's cut into quarters and cut up fruit, as well as crackers and cheese slices. From my experience, kids will be itching for the party games, cake, and their treat bag, not for what food they'll eat.
There are so many more possible themes you can imagine. But, those are my favorites, especially since I love any occasion where we can dress up in costumes. A couple of websites I really like for costumes and party supplies are http://www.dizguise.com or http://www.hullabaloocostumes.com. Hope my post starts you off on some party theme inspiration! Good luck!