Great question! It is obviously difficult to teach a lot in one hour and I am guessing that you will be hands on based on your suggested approach. I think that I would try and teach them something basic about the sport and try to incorporate it in a fun way. Try something like this, divide the group into two groups. Position on set of the children behind second base and the other behind homeplate. You can make it a relay race. Give each team a baseball. The player runs to the next base so from home the player runs to first, before getting there they have to yell the name of the base. Continue until complete. When you are done you may have taught them the names of the bases. I would then teach them the basic mechanice of throwing. I use a saying that when you throw its thumb to thigh hand to sky. Helps teach or enforce the windmill throwing action. The last thing that I would go over with them is fielding. Again I use a saying that goes, Elephant, alligator, belly button. The arm with the glove is the elephant the 'trunk' is on the ground ready for the ball, the throwing arm is alligator. As the ball is entering the elephant you trap it with the alligator then both hands raise together to the belly button. Hitting is very complex and I would think that one hour is already used up. If you do venture into hitting, a simple set up and swing motion would be enough. I hope this helps. and good luck