Tennis Soccer is a simple but effective activity for your youngest beginners. You will need four cones or spots to mark the 'goals'.
Two players, or a player and a coach, face each other on the same side of a 36-foot court. If you are using a full size court, have the cones at the net and the service line. Each has a goal marked by two cones. Make the goal about 10 feet across. One goal is at net. The other is at baseline. Players take turn rolling the ball back and forth to each other with their racquets, trying to get it past opponent and in between cones for a goal. First to 7 points (goals) wins.
Here's a video of the activity in case this description doesn't do the job.