I have added some songs to the Gruffalo. They are ROCK classics. You can find backing tracks and lyrics sort of karaoke on Youtube. (There is a link on the script) These are relatively simple songs and your children will have fun singing and if possible playing along. I share out the narrator parts to give more children a speaking role. I have done this in assembly quite a few times now, always seems to go down well it takes about 15-20 mins.