I get the feeling that you just mean a party and not a sleepover....not sure if I am right?! At that age the kids like to just wera nice clothes and may be confused about wearing PJ's to go to a party...I think sometimes we try too much!
The best party I ever did was for my daughter when she was 6...all the parties we went to were bouncy castle parties, ball pool parties, splash pool parties, jungle gym parties etc, so I hit on the 'novel' idea of holding a traditional birthday party...proper games, little sandwiches, jelly and ice cream etc, and the kids LOVED it...even the mums admitted they had not been to a normal party for such a long time. We are so busy trying to come up with new and exciting ideas that we forget about the tried and tested formulas that have worked for us in the past. xx