Here's a suggestion from an AMA contest director. Turn out at pattern contests has been low for about 15 years, so we have to come up with creative solutions since we have less money from entry fees to work with. I designed simple certificates in PowerPoint with the name of the contest, date, and location. I leave space for a 4X6 photo. I export the certificates file to PDF, so the fonts become platform agnostic and take them to my local Pak Mail and have them printed on card stock.
During the contest, I take a photo of e very pilot with his/her airplane and have the photos printed at Walmart, CVS, or Walgreens. As the final standings become known, I use double sided tape to attach the photos to the certificates and place each in a frame from Walmart.
If you like, I can design a certificate for your kids and leave a guide for a photo of each kid with his/her rocket.
Rob