Line up four poly spots from the net to the baseline as follows:
one at the middle of the service court, one at the service line, one half way between the service line and baseline, and one at the baseline.
The coach is on the side opposite the line of spots, the young player starts from the spot closest to the net.
The player is required to rally two balls in a row with the coach, starting and recovering from the first spot.
If he succeeds, the player will move to the next (further) spot (the one on the service line), where he will try to rally three balls in a row (recovering to the same spot).
If he makes it, he will move back further to the next spot (going for four balls in a row).  If he fails to rally the required number of balls, he then goes back to the previous spot.
The player attempts to move back one spot at a time until he gets to the last one, on the baseline.  From here, he will try to rally five balls in a row with the coach.

Note: This is a good game to develop consistency under pressure for beginning players.