Here's the first of several drills from the recent USTA South Carolina coaches workshop at Cayce
Lauren's presentation was about drills for womens' teams of various levels. But I found all could easily be adapted for junior players.
Lauren had her drills organized by playing level: 2.5, 3.0-3.5, and 4.0. She said 2.5s often struggle with or avoid hitting their backhands, so she suggested this drill to build skill and confidence.
Partners pair up across the net from each other, behind the service line. One player may only hit backhands. The other may hit any shot they like. Players rally straight ahead. Ball must land in service box. If the designated backhand hitter hits something other than a backhand, that's a point for the other player. First to X points wins. Round Two: switch to hitting cross court.
Progression: move farther back or switch to a faster ball. For examply if you are hitting orange ball, change to green.