Hmmm...maybe you should just go with the invitation and have a cupcake/candy party...both boys and girls love cupcakes and candy so gender wouldn't be an issue.
For decorations all you'd really need is different colored balloons, tableware (plates, napkins, cups, utensils, ect) and assorted containers filled with different types of candy.
For food you could get the bread that has swirls (kind of like a pinwheel) and make different kinds of sandwiches. I wouldn't think it really matters what food you serve. Instead of a traditional cake, I think a cupcake tower made of simple sprinkle cupcakes would be great!
If you do favors you could simply get bags and have the kids fill them with the candy from the table.
This could be a really fun (and simple to put together) party!