Tested this, and you're right, but if you drag and drop each item you want in the dope sheet, it only shows the keyframes from that object (parent, child, doesn't matter). It'll only show all keyframes in the 'summary' at the top.
Then again, this might be confusing because you can't see which one is a child of what in the dopesheet (you can however if you look at your hierarchy in the objects manager.)
I also didn't know about those layer colors in the dopesheet, so thanks! Unfortunately I never use layers. :'D