There are two popular games that are played at my club during both the youth and adult classes:
1) Fencing the line - there's a large box on the floor, divided in two by a line. Two teams face each other across the line, and the object is to score a proper touch on someone across the line (we use foil target area). You can step over the middle line with one foot, but if you step outside of the box or over the middle with both feet, it's treated as a hit by an opponent. You can gang up on a person across the line - usually one or two people will keep someone's blade busy, and another goes for the touch.
Once you're hit, you leave the box, walk around (this is important for safety reasons, don't want anyone getting hammered in the back, neck, or head), and join the team on the other side. If you're hit again, you're out of that particular round.
This game is pretty popular at my club - it's even more interesting when a coach is on one of the teams, and is fencing against three people. Neat stuff.
2) The Three Musketeers - I think this is what it's called... anyway, if your class is big enough, divide it up into four groups of three, and put one group in each corner of the room. When the game starts, each team moves more or less together, trying to score touches on other teams and be the last team left. Any team member hit is out of the game immediately while their teammates continue on. The last part of a team or full team remaining wins. It has to be stressed that any attacks must be made from the front - this doesn't mean that you can't wait behind someone for when they turn around, but you can't attack from behind.
This may not be as good a game to play with little kids - some of them may get too into it and hack away at each other from behind.
One other thing we sometimes do when doing footwork drills is for the coach to give hand signals for different actions, as well as verbal commands. The idea is that the hand signals take precedence over the verbal, and sometimes conflicting commands are given. Rewards are kind of difficult to figure out, but maybe whomever is the last person to not make a mistake (always followed the hand signals) gets to pick their partner and gets to use the electric gear first?
I hope this makes sense (I'm pretty tired) - if it doesn't, let me know and I'll try to get my thoughts together a little better.
