Calm Kids​
When my kids are sleeping, they breathe with the ratio of the golden ratio. You can use this music to help your kids relax. You may also make them aware of their own breath. If they are able to breathe with the music, thats good, but do not force them. Just make them aware the music has a breathing pattern. The slow tempo and the safe ambient sounds will hopefully calm them down. 
Or just play it in the background and see if anything happens to the mood.
These tracks are just available on this site, and they are available for streaming the entire tracks.