We also had PJ days at my school. Everyone wore them. Make it a silly party, and the clothing fits in. You could do some crazy things like wear those foam rollers in your hair, have the adults in bathrobes, and so on.
I will say that 12 is a lot of kids for a 6 year old party. If you're already committed, fine, but try to scale things back. It can get expensive and unruly, and then of course you have to go to the parties that all the other kids have through the year.
You could give out related favors (if you are doing that) like night-lights or those glow-in-the-dark stars and comets kids put on the ceiling in their rooms. I don't believe in going overboard with that stuff, but if you can find it at reasonable prices, it might fit in with the theme. Try the dollar stores.
Have fun!