I just had our first slumber party last weekend for my daughter who turned 6. We had my two girls plus 4 others, and it was a little crazy. My best suggestion is to have one other mother or friend of yours to do it with you. That way when everyone is asking for something different at the same time, you have four hands instead of two. It will help keep your sanity!
Our schedule went like this: everyone arrived about 7pm (after dinner). We blew up balloons and put candy in them, then the girls sat on them to pop them and kept the candy. Then some did water paints and some watched tv. They played a little (still at the age where playing with toys is cool!) while I finished decorating the cake. We had snacks (mini marshmallows--which was a big hit!) and got ready for bed. It was the first sleepover for all of them so we went to bed around 10pm and the last one fell asleep just after midnight. They woke up about 7:15am, had mini donuts and milk, got dressed and watched cartoons. We left the house at 10:30am and went to play at Bullwinkle's. They have a large play structure similar to Discovery Zone, only huge! It was $5 per kid, and a big success. Went home at 12:30, ate lunch, had cake and ice cream and was done at 2pm. For the ice cream I used an idea from the local ice cream store and made clowns out of a scoop of ice cream, sugar cone for a hat, 1/2 cherry for the nose, and frosting eyes and hair and mouth. It was a huge hit! I think if they were older they could have made the ice cream clowns themselves. They were super easy. I hope I have given you some different ideas!