Have children jam together using quality, sturdy instruments. Instruments that are appropriate for kids include xylophones, bongos, recorders, kazoos, small guitars and ukuleles, maracas, harmonicas and any other instrument that isn't extremely expensive, breakable or potentially dangerous.
Adults present at the kids' jam can provide a steady rhythm or melody, giving the music a more satisfying sound until, with practice, the kids are able to create a cohesive sound in their own.
Recording the jam session for later playback can be instructional and exciting.
Have a group sing-along. Get song ideas from a children's song book, a recording of children's songs or from the kids themselves. Take turns leading and teaching one another the songs. Some songs can be accompanied by dances or sign-language, and others can be sung in a round or with harmony. Don't underestimate the kids' capability to pick up on complex songs. The kids can organize a performance of their singing once they are comfortable with a number of songs.
Have kids divide into groups of four. Each group is then responsible for coming up with a song. Suggest that the song may include verses and a chorus; rhythm, melody and harmony; a message or a meaning; dancing; or audience participation. Have the kids pick a name for their song and for their songwriting group. After all of the groups have made up a song, hold a small performance by all of the groups for one another or for the parents and guardians of the kids.
A fun music project is building instruments at home. Not only will the kids end up with real instruments to take home, they will also gain knowledge about music theory and physics. Some fun instruments to build include shoebox guitars, kalimbas, used bottle xylophones, whistles and all sorts of percussion instruments.
It can be fun to invite a musical guest to play for and teach a bit of music to a group of kids. For younger kids, invite someone who writes children's songs specifically. A good performance for younger kids will involve interactive songs, and kids and adults alike will come away from the performance with several new songs in their musical repertoire. For older kids, invite a professional local musician who can play a few songs, answer questions from the kids and teach them a few tricks of the trade.