As unbelievable as it may seem, I am 99.99999999% certain that the adult voices WERE the original tracks that the characters were animated to. AFAIK, I was the first person to notice this; before I did, nobody else online had ever brought it up.

Look at the one with the stocking cap who says "Forget it, you're one of us now!" in the original released mix. Then look at the remix, where he says "Forget it, you're junk, you're trash!" I watched the two versions side by side...and the adult voice in the remix was the one which matched the mouth movements.

My assumption is that there was a decision during post-production to redub the voices with actual kids (and change a couple lines), or that the adult voices were a temp track never intended to actually be heard in the final mix. (I also have a feeling that they were Bluth crew members - which would explain why their acting is on par with that of the laserdisc games.) I even wonder if the off-screen "Pitiful!" heard in the '86 mix was an ad-lib by the child actor.

The 5.1 remix was assembled using the original dialogue stems (including lines that were recorded, but dropped from the theatrical mix), and so it would seem that these adult voices were the ones on the stems. As timdiggerm has suggested, maybe the child voices couldn't be found, but I think it's more likely that the people who put together the remix didn't know the voices were changed for the final mix (or worse, didn't care).