When we say "science fair", we're talking about an event at which students present projects that they've carried out, usually standing in front of trifold poster boards being interviewed by judges. It seems like you're talking about an event at which outside people come to the school and give demonstrations of science/engineering concepts -- is that correct? If on the other hand you're looking for people to mentor students who are completing their own projects in these areas, this might be useful: http://www.sciencebuddies.org/science-f ... tors.shtml
The first thing that comes to mind is an outreach program that my undergrad dept does, which is unfortunately only an option if you live in central VA: http://phun.physics.virginia.edu/
or this club at my current university, which does demos in central PA: http://www.clubs.psu.edu/up/sciencelions/
If you're outside of those areas, it might be worth emailing them to ask if they know of any similar programs in your location. You could also contact nearby universities. Physics, mechanical engineering, and electrical engineering are probably the departments most relevant to robotics/mechanics, and 'outreach' is a good keyword -- but failing that, the department secretaries generally know everything.
For something a little more specific to robotics... this might be the Lego thing you mentioned: http://www.usfirst.org/roboticsprograms/
What you could do is try to find a high-school-level FIRST team in your geographical area and see if they would come to the elementary school and do a hands-on demonstration as an outreach project.
For safety/privacy reasons, we discourage students from posting any identifying info such as their locations. However, in your specific case we could probably give better recommendations if you wanted to tell us what state (or country?) you're in.
Hope that helps,