Small children can become confident composers of lullabies. A good start is to practise rocking movements with the class doll or teddy bear, and feel the gentle rhythmic simplicity of a song such as the traditional Scots "Ally Bally Bee".
(For a hilarious contrast, try "rocking" the teddy to sleep with a jig or reel; the children will volubly explain why it isn't suitable as a lullaby, using aptly descriptive language about dynamics, pulse and musical purpose.) Then ask children to make up their own sleep-inducing melody on a xylophone, using the rhythm of the"Ally Bally Bee" refrain as a starting point.
Appoint two children to say whether the teddy bear wakes up or remains peacefully slumbering - an unfailing inducement not to play too loud.
Amplify this by listening to Debussy's marvellous "Jimbo's Lullaby", a piano piece written for his daughter's toy elephant, as a reward.