Dreamwork's How To Train Your Dragon is the perfect theme for any age!
If you don't have room in or outside your home to host a lot of children, think about finding the perfect venue. You are going to want to find a kid-friendly venue that may have some sort of outdoor area to allow the children to join in on outdoor party games. You can even rent a water slide or a bouncy for them to enjoy. On a side note, it helps the children relieve some of the energy they have, and saves you from a lot of indoor noise.
When looking for a venue, think about a bowling alley or local pizza place (if you don't mind skipping out on the outdoor fun). Those sorts of places will assist in the decorating and possibly provide balloons and other party decor. It's not going to be cheap. so you may want to re-think the party at home.
Once you've decided on a place to host your party, it's time to get those invitations out. Depending on the age of your child, you could sit them down and have them make cute invitations for all their friends. You can even have them make one full-sized invite and print out as many copies as you may need. They can do it at the table (you will need to scan the finished product) or they can do it with software or online at a site. Either way, it will give you more time to decide on a cake!
As far as paper supplies for your party, if your dragon cake is green, simply buy napkins and plates that match or accent your other items. You can do this with balloons, as well. If anything, it will save you a bit of money in the end.
(Note: There's excellent concept pages, full of unique and exciting "Dragon Party-themed" party decorations! Check them out on Pinterest!)
As far as other decorations, you can be as creative as you like. The cake can be a mountain lava cake, with tiny dragons around the base. There are plenty of party stores that offer " How To Train Your Dragon" supplies, and just by stepping in the aisle, your creative side will kick in, and before you know it, you are loading everything you need into your trunk, with a smile.
Party Favor Ideas: Paperback books from the movie. Dragon erasers, pencils, and so on. Look for these at local party stores or online.
Food Ideas: Dragon Balls (Meatballs with food coloring). .. and so on! Be creative, good luck, and have fun!