Hm, metaphorically speaking, that's probably what Don Bluth was trying to get across.
It's not really that the kids died and found this "Great Valley". However, through their pain, troubles, and such during the journey to the Great Valley, it was probably reflecting on how every person has to go through a journey in life and then find your heaven, or in this case, "The Great Valley", when it ends.
I'm not saying this is EXACTLY that this was the moral that Don Bluth was aiming for, but it could be possible that when he made this film, he was reflecting human actions and how everyone has their own journey to go on and then everyone finds their "heaven" or "eternal bliss" or whatever you want to call it.
I'm also not saying that these "rumors" are correct either. They could have been made up by some LBT haters that just want to make trouble...-_- I did have to laugh at that rumor though...I mean honestly, that wouldn't be good at all, kids would be crying, parents would probably be enraged, and some animation fans or Don Bluth fans would probably go: :blink: