If kids are driving it then aside from the fact that they can hit stop at any time, a parent should not let them into an unsafe situation. When it is just the robot driving and is in autonomous mode, then it depends on what terrain you have it set for. If it is on a fairly flat consistent surface you can monitor the part of the point cloud from the kinect that you are expecting to be returned from the ground area in front of the robot. You can adjust thresholds there to exhibit specific behaviors depending on the terrain. ie No return signal in front of robot, then make STOP! behavior.
If it is on rough terrain then it gets a little more complicated to keep it moving forward. You have to get a little more constructive with your code and figure out how to deal with those situations, that is the fun of building and programming robots. Some other issues present themselves when you solve one problem, if you monitor the point cloud and it disappears so you stop, how do you make the robot go over a hill. When he gets to the crest of the hill he see’s no ground and would benefit himself to stop and evaluate the situation, but then how do you overcome it? Lot’s of fun programming! 🙂