|Sometimes it's hard to tell|
who is in charge
Two players play singles against each other, beginning on respective deuce halves of court. One player is Master; the other is of course Servant. Coach feeds first ball to Master. Master must hit shots to Servant alternating between deuce and ad sides. Servant may only hit back to Master at deuce side. First player to 15 points wins. Switch roles. After both players have had a chance to play both roles, switch to ad side of court and repeat.
As you have probably figured out, the Servant will be doing a lot more running!
Easier: for young beginners, have them tossing and catching the balls rather than hitting with racquets.
Harder: require X number of rallies before points are awarded.