In a 2011 article for TennisPro magazine, USPTA and PTR Master Professional Ken DeHart breaks down the basics of returning serve. He divides the receiving court into four blocks. Deuce service box is 1; area behind deuce service box is 2; area behind ad service court is 3; and ad service box is 4. He suggests discussing receiving/returning strategy with your doubles partner ahead of time so the net player has some idea of where the returner's ball is going and therefore how to defend subsequent balls.
Pre-planning and defensive doubles strategy may seem a little too advanced for young players, but it is never too early to ask them to be able to direct the ball to specific parts of the court. So I have devised a simple activity based on Ken's receiving principles. He mentions boxes 2 and 3 should be your fallback position if you are struggling on the return, so we will place a higher value on hitting the return into the deepest part of the court.
For the activity, you will need an assortment of cards numbered 1-4. If you are only using 4 cards, recycle them so that they may be drawn multiple times. Ideally you will have at least two players; one serving and one returning. Returning/receiving player draws a card before each return and must hit their return to that box. Player receives two points for each successful return to boxes 2 or 3; one point for each successful return to 1 or 4. Serving player receives one point for every miss. First player to 11 wins; then switch roles.