[Katie_UPS]

If you have four groups, then you have ten minutes (a little less) to "entertain" each group, which seems to be reasonable.

1) actually explain the team, allow for Q&A... you need to have information somewhere anyways
2) Drive/see the robot
3) I second the human-minibot races. Maybe a mini-obstacle course? Obviously you need to make sure they won't damage the minibots though.
4) Show old robots and old game animations/videos? Build season in 6 minutes (game animation + coming up with robot ideas)? Something else that is short in time but still somewhat entertaining?

Don't kid yourself on how much time you have. 40 minutes with 70 kids? You're going to lose at least five of those minutes with organization and switching tasks. You also don't have tooo much time to give each kid a hands-on experience in a one-at-time activity (just a thing to remember).