Grouping does not appear to work !!

Hello,

I have a hollow cube object which has two other objects inside. I’d like to animate the cube falling and smacking on the floor using Rigid Body physics, with the two inner objects following the box animation as if fixed inside. Unfortunately, I can’t seem to get the animation to work. If I parent the objects, the animation causes the box to flip around and circle the floor erratically without end. Grouping does not appear to work either.

I’ve confirmed that if I use the Rigid Body settings for just the box, it falls exactly as I’d like. There must be something simple I’m missing, any ideas?

Please help.

I didn’t find the right solution from the internet.

References:
https://www.blender.org/forum/viewtopic.php?t=21043
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Thank you