Tell them the goals must be scored by shooting on the ground. After one minute goals and players switch. Don't worry about the finer points of throw-ins, offsides, etc. Do prohibit sliding tackles, encourage the attackers to shoot, defenders to get back as soon as they lose the ball. Defenders are everyone on the team that doesn't have the ball.
Attackers are everyone on the team that does have the ball.
When noone has the ball, deciding whether you should act like a defender or an attacker is hard to determine, but the team that gets it right most often usually wins the game. The scrimmage will likely look like a swarm around the ball. If the coach must engage in some tactical instruction, have one player play behind the swarm to collect any balls coming to him and play the ball forward to space in front of and to the side of the swarm. Later introduce players to the sides of the swarm to collect any balls to the side or passes from behind and then dribble forward and shoot or pass to the middle. Finally, add a player in front of the swarm to serve as a target.