Nutritional yeast & kelp powder were staples in my house growing up.

Garam masala! Old Bay! Cinnamon and sugar! Bacon Salt! The powdered cheese from Kraft Macaroni 'n' Cheese! Hot sauce! Brown sugar and chili!

Cinnamon sugar, mini chocolate chips, dried cherries/cranberries, raisins (yogurt & regular), pumpkin seeds, mini M&Ms, gummy bears - I have used them all for "make your own" kid parties. They should work for popcorn too!

I was thinking Quik would be kinda tasty. :)

Butter and furakake seasoning.

I love parmesan and black pepper on my popcorn. I second the mini m&m's and chocolate chips too. You might also want to have a misto with some olive oil or some other light tasting oil to spritz on the popcorn to help the seasonings stick.

I also love curry powder on popcorn, but that may be a bit sophisticated for the kiddie crowd.
To that end though, you could throw in raisins and coconut.

herb/spice mixes like poultry or veggie seasonings are often really good on popcorn - I use Penzey's mural of flavor and my friend has a 'bread dipper' spice blend that's wildly delicious with popcorn.

Target sells a buffalo chicken flavored popcorn seasoning that's also super delicious (if you like buffalo chicken).

On the sweet side, in high school we would put milk duds in warm microwave bag popcorn whenever we had a movie night.

I second the za'tar. Definitely.

I second the garam masala and Old Bay. Also recommend Chinese 5-spice powder, Pho seasoning, ranch-flavoring packets (the kind you add to yoghurt to make ranch dressing), peanuts, wasabi.....probably not tasty for kids but these are the flavors I make at every gathering in my Whirly-pop.

there are great ideas here:
http://www.etsy.com/listing/83743522/gourmet-spice-kit-for-popcorn-delicious

GREEN TABASCO SAUCE! It's seriously amazing. It makes the popcorn a little soft, since it's a liquid, but I first tried it at an American Irish pub and it's fantastic.

Maybe not the best suggestion for kids, but I grew up drizzling honey on top of popcorn. In Hong Kong, where my mom grew up, popcorn was a sweet snack, not a savory one.

I like to use Trader Joe's Everything Seasoning on my popcorn - it comes with chunks of spices in one of those pepper grinder things. Using the grinder could be fun part of the make your own party. The store has a few other spice mixes in grinders too that could be worth experimenting with on popcorn.

Not sure how old your daughter is, but my favorite popcorn topping is Sriracha mixed with melted butter or olive oil.

have you seen this post on luxirare?

http://luxirare.com/popcorn-bar/

it's a slightly more grown up version, but might give you some ideas!

Every time we have crab legs for dinner, we melt way more butter than we need then use the leftovers on popcorn (after having dipped the crab bits in). The best popcorn we've ever had by far!

All of these ideas sound delicious! I never tried this, but since this is a kid's party, maybe some sprinkles? They don't add much of a sophisticated flavor, but the popcorn will look so pretty and festive.
I was recently treated to some homemade popcorn mixed with freshly grated parmesean and black truffle oil and salt. Oh my god, foodgasm.