Why don't you use a winter theme? I know it sounds corny. But you could do so much with it and it would be great for kids here in texas that doesn't really get to see snow.
1. covered the whole room in snowflakes that you cut out of paper.. hang them from the ceiling.. everywhere.. let your daughter glitter some of them. You could even cut out huge ones and huge letters spelling out happy birthday to make a banner for them.
2.You could have games where you lay out layers of winter clothes like hats, mittens, jackets, boots and the kids take turns seeing who can put on all the clothes the fastest. let two kids go at a time.
3. Let someone make a cake that looks like it has a icy road on one side and a ice skating rink on the other.. Put a iceskating doll on the ice for your daughter and a rugged truck on the road for your son.
4. cover blocks of wood with regular house socks. Rope off an area of your kitchen or living room if you have wood floors and let the kids pretend to ice skate while standing on the blocks.
5. make smores using a fire pit or even the microwave
6. get some fake snow and put it in a huge tub for them to play inside with. Put in cups and other things for them to dig in it with. If cost is not an issue, cover an area of the yard outside with the fake snow and suprise your kids with it.
7. you can find some many awesome crafts online or at michaels using the foam that the kids could make and take home as their party favors.
8. make snowmen using marshmellows. let the kids decorate them with candy and icing.
Sorry for some many things but I love to plan parties!! I'm always up to give anyone ideas...