Centroid of child components in Assembly

You could:
1. evaluate the centroid of each component in the context of it's part studio, using the evaluated featurescript endpoint.
2. Translate that centroid using the transformation of the component in the assembly from the getassembly endpoint

If you are trying to find the centroid of all child components collectively,
3. You could then sum all of those centroids from #2, weighted against the mass of each component.

If you are looking for a featurescript only way of doing it,
You could make a featurescript that takes in bodies and calculates the centroid, and just run it in an incontext part studio in the context of the assembly. This is probably the easiest way of doing it.