Check out the UCMP web site at
http://www.ucmp.berkeley.edu/museum/k-12.html
for activities, especially the on-line book "Learning from the fossil record". One of the amazing things I learned from that exercise was that teachers and students about the age you note were transfixed by microfossils. If you have some microscopes and some disaggregated sediment, you can keep them busy exploring for at least 1/2 hour, maybe the full 50 minutes.
Jere Lipps