decompose does not work that way it requires parameters for the decomposition to be stored in. See Babylon.js Playground
As usual please supply a simple playground showing your code and what is wrong when asking a question. You will usually get a quicker answer, for one reason it is far easier for a replier to add a correction to a PG than to construct one and so more likely to get a response.
Thank you for the help. I will make a Playground next time.
I turned your solution into the function below and it worked. I originally asked because I was surprised there wasn’t a built in solution like object.absoluteRotation.
let getChildRotation = function(child){ //return the rotation of a child of a parent object by using a temporary World Matrix
var scale = new BABYLON.Vector3(0, 0, 0);
var rotation = new BABYLON.Quaternion();
var translation = new BABYLON.Vector3(0,0,0);
var tempWorldMatrix = child.getWorldMatrix();
tempWorldMatrix.decompose(scale, rotation, translation);
return rotation;
}