The kids line up in two single-file lines behind the baseline: one on the right, and one on the left.
In front of each line, at the base of the net, there is a pile of balls (see graphic below). Each pile has an equal number of balls in it.
At a command, the first player from each line will run to their respective pile of balls, pick one ball up then run back towards the service line. Once they get there, they will stop, and execute a serve (or groundstroke, previously determined by the coach).
If they manage to hit the ball over the net, they will run back to their respective line and tag the next player who will also run to repeat the same action.
If the player, however, hits the ball in the net, they will pick up the respective ball and try again. They will run back to their line only after they successfully hit the ball over the net.
The kids take turns one by one (from each line) until a team hits all their balls over the net and the last of their players passes the baseline before the other team.
Note: 
This is an excellent game for beginner players who are still developing their strokes and enjoy a good workout in a team format. We recommend this game to be played at the end of the lesson; kids will be cheering, and leave your tennis class happy and... tired. :)