To start, I would pick up some card stock and make your own invitations. Be creative with this- maybe trace your child's hand and hand-write the party details. Show the crayon theme as opposed to stating it by using crayon as the only medium on the invitations.
For decor, find a large roll of craft paper and use it as your table cloth. There is nothing more fun (for children and some adults) than doodling on the table with crayons. Use crayons as your centerpiece - try using clear glue and rimming a low clear vase with them. Then fill the vase with the crayons for use on the table. Also, tie up a bundle of crayons (maybe make these a special edition like glitter crayons or special colors) for a favor at each place setting. To help fulfill all these crayon needs select a larger quantity pack such as this 400 piece crayon set from Oriental Trading.
Hang large crayon shapes such as the ones picture above to add to the room's crayon decor. (image via flickr use Kid's Birthday Parties) Can't find these? Make your own- this theme should be all about creativity and artistry!
Dress the birthday girl in this adorable crayon carrying apron.
An excellent idea for special crayons are these 'crayon cakes'. No, they're not edible but they are super fun to use and easy to make at home. Baking tray image, finished crayon cake image and idea all found at Providence Homemade. These will certainly add to your table display too!
Lastly, as far as food think anything with multiple colors. Maybe make a candy buffet based on the primary crayon colors- M&Ms are perfect for this. For dessert, ask your baker if he/she can make a crayon themed cake or use a homemade favorite and top with these crayon candles from Everything But Cake.