Remember that not every student should or will wish to enter a Science Fair. It is not for everyone. Encourage all to enter but recognize that many will not. Every child can contribute to the Fair in some way. You might conduct a mini-Science Fair in your own classroom to encourage participation and build confidence.
Every child can potentially contribute to a Science Fair.
For those that wish to enter remind them that there are guide-lines to follow. Leaving things to the last minute is disaster. Things will go wrong and you will lose time. However long you think it will take to conduct your project, it will take twice as long. We have found this to be almost universally true.