Well, for every concerned parent a healthy diet is essential. When it comes time for the birthday party, one where a group of kids will be playing at your house, there definately has to be a line where you shouldn't cross as far as the treats go. It is once a year, a party so it is alright for the children to have some "junky" food but you must try to find some alternatives.
Kids love watermelon.
Other fruits may be appealing to some.
Wacky foods that they have never had before may be fun.
Be creative, make healthy foods entertaining.
Trail mix, the healthy version.
Your not trying to impress your peers, so don't have an exotic setting. Just have something youthful that will make the kids have a longer attention span on the setting. Not only that, but they don't neccessarily want junk food, they just want something that isn't plain and boring. A lemon in a glass of juice may be ultimately better then pop or chocolate milk for them. Try switching the salty potato chips for some banana chips.
There are loads of different ways to keep your children on a healthy diet on their birthday, just be creative
Don't forget to have fun while doing this too. I look back to my younger birthdays and I think my mom had more fun then me.

Today is my birthday anyway