Parent child relationship is lost when physics are applied


I am a total noob to Blender, so please excuse if this is a total beginners question.

I have made two spheres, one inside the other. I made them parent and child, the inner sphere being the child. Now, as expected, I can move both of them when only moving the outer parent sphere. So far so good.

Now, I want to use the two spheres as particles: I choose to use a collection containing both objects and check the “whole collection” box. The particles are created and the child spheres sit inside the parent spheres.

Finally, I convert the particles into objects and try to run a rigid body physics simulation: the former particles are supposed to fill a large sphere. I use gravity to collect the objects on the bottom of the large container sphere.

Problem: now, the child spheres pop out of the parent spheres. The child parent relationship is gone.

What can I do to keep the child spheres inside the parent spheres?

Thank you very much for your help!