In the U.S. something becomes prior art under 35 USC section 102
(1) the claimed invention was patented, described in a printed
publication, or in public use, on sale, or otherwise available to the public before the effective filing date of the claimed invention; . .
Sending in the registration would not likely be considered a publication but if/when is printed in a guide to the fair it would be a printed publication - even if only online. Would it be considered available to the public? That is a good question for the patent bar exam so I do not recommend counting on it not being "know to the public".
Of course it depends on the level of detail you put in the disclosure. If you only explain the result or purpose and do not describe the structure that achieves that result you have not disclosed the invention. The U.S. does have an odd 1 year grace period. As another answer mentions you can file a provisional patent application with the USPTO. One idea is to file the exact same thing you use for the fair registration as the text of the provisional.