We had one at this same age. We told the little girls to bring dress up clothes and that if they didn't there would be some they could pick from.
We took dress up shoes, mine of course, long dresses, bathrobes, old Halloween costumes, etc...anything that could make a true princess feel right at home.
I took some fabric that could be cut up for the boys to make capes. I let their moms know if they wanted me to find something in particular that I would try.
I bought some foam crown shapes in the kits at hobby lobby. I had an old tackle box full of glue on gems, google eyes of many shapes, colors and sizes, glitter, tiny tinsel, bling, you name it, I had it in that tackle box.
I did use some faster drying foam craft glue and it worked very well. So here it was, each child got to dress up as a member of royalty, make a custom crown, and have their picture made in front of a wall hanging of a path that led to a truly royal palace. I think I got it at the party store for just a couple of bucks. It might have even come from hobby lobby too.
I think you can do many things with a party of this theme. I know the party stores had numerous items to go along with it. The kids can play dress up, sword fights with foam pool noodles or those foam covers for pipes in the winter, anything soft that won't injure. There are just so many things one can do.
Have lots of fun!