Once a month switch up the lanes. Just for that practice. It helps keep swimmers mentally fresh and on their toes.
Try a snake drill. Swimmers form a line in lane one and swim up and down each lane (for however many lanes you might have) like an "S". Each lane do a different drill.
Play a game that incorporates swimming sets or drills. I like to play The Dice Game. Each number a different set or drill. Six is a challenge. One is fun.
It is important as a coach to remember that for swimmers- under 12 -the emphasis at practice should be a balance of anaerobic and aerobic work- with a focus on stroke proficiency and not yardage.
Physiologically, tween swimmers do not need as much yardage as older, postpubescent swimmers. In fact, too much yardage increases risk of injury, burnout, and, according to a swimscience.net article, "Studies show over-training before age 14 can actually reduce the chances of later success."