S.B.
I think I'd focus in on Abby's flying fairy school to incorporate something more "boy" friendly. I imagine you could print some pics of Gonnigon and Blogg if you can't find decorations. It shouldn't matter, but maybe it'll make Grandma and Dad feel a little better. Play sesame street music as the kids come in.
We were invited to a fairy party. The hostess had fairy wings and wands to decorate. Most of the boys had no interest in the wings, but they did enjoy the wands quite a bit.
Fairy School is all about problem solving. You could do a simple scavenger hunt with rhyming clues similar to the spells they use...."splish, splash, splace, look where you wash your face." In the bathroom there would be another clue. The kids could help each other out and you, of course, could guide them in the right direction. When they get to the bathroom sink there could be another clue. etc. etc. The "treasure" could be their goody bags.
You could also make plain cupcakes and let them use "fairy dust" (sprinkles) to decorate them.
You could play Pin the Wand on Abby...whoever gets it closest to her hand wins.
Face painting is a hit no matter what the theme!
Sounds like a cute party. Have fun!