Well, I am double 3-1/2-in insulating my entire bedroom seiling (400
sf) and then sheetrocking the whole thing (and then maybe hanging
suspended celing to boot). My 7-year old son plays upstairs in his
bedroom every morning before school, and I know how loud he is, so I
will report back once the insulation is in place. Just this morning he
was commenting on how he could hear mommy and daddy talking before we
went to bed, and his recollection was stunningly accurate. We'll just
see how much he can hear after I'm done...

Instead of just sheetrocking, put up a layer of asphalt roll-roofing
first, then put lathe crosswise to the joists, and put the sheetrock
up on that.