Holidays can provide you with several options for musical set design themes. For Christmas, you can have imagery of trumpet players and drummers playing in the background. Halloween can evoke imagery of skeletons playing xylophones, or using each others ribs to play. St Patrick's Day themes can include images of harp players and Irish traditional ensembles, complete with a bodhran. Valentine's Day can have images of cupid playing a violin or singing to a couple in love.
Animal themes work well for children. If you are putting on a production of "Peter and the Wolf" by Sergei Prokofiev, you can indicate the musical instruments that each animal is represented by. The bird is represented by a flute, so have a bird carrying or playing a flute mid-flight. Have a cat holding a clarinet. The duck will hold an oboe, while the wolf could be seen chasing the duck and playing a French horn. The human characters include the grandfather on the bassoon, the hunters on timpani and Peter could be conducting a string orchestra.
A nature themed musical could have images of drummers playing deep in the woods or a Celtic festival hosted in the mountains. Take a subtle approach and draw musical notes in the air, or create trees in the shape of a bass or treble clef. On the actual stage, set up musical instruments, a music stand instead of a podium or a conductor's podium in the shape of a rock.
There are several musical toys that you could include as props or part of the stage backdrop. For the backdrop Include images of maracas, drums, trumpets, shakers and children singing. On the actual stage, design a large war drum to sit in the corner of the stage or have xylophones placed around the edges. On the backdrop, paint an entire orchestra and make it appear that the children are attending an orchestral concert.