Did anyone explain to them the magnitude of devastation they carried, or were they just told it was a large bomb. They had to have some idea since they only had the one. I can imagine they had a hard time understanding what it was capable of until watching it a aftermath, since nothing had ever come close before. Is there any record of their permission briefing? I would think the pilots may have known more then the crew. I guess need to know could have come into play. Can't imagine what it would be like to try to wrap your head around a completely new weapon like that.
Sadly I don't have direct sources but I'm citing Pacifc War 1941-1945 pg 586-591 by John Costello. The book has proper citations, but this is a quick recap as I read it.
Some high ranking officers were all aware of the power of the bomb, but only on a strict need to know basis. The crews dropping test bombs were unaware of exactly why they were doing so, only that it was to practice for a new type of bomb. Once somebody was debriefed about the actual weapon, they were deemed too essential to risk capture and interrogation and were forbidden to do anything that may risk capture. So once briefed you couldn't actually practice dropping the pumpkin bombs anymore.
Curtis LeMay (Head of the Pacific Air Forces, I believe he was on Tinian while the bomb was present) and Paul Tibbets (Ranking officer and pilot of the Eonla Gay) were aware as well as other similarly ranked officers on Tinian. Additionally the engineering team that assembled Little Boy on the island after transport knew what they were creating. That was it.
Even the ship assigned to take Little Boy (The Indianapolis) didn't know what the cargo was. Only that if the ship were to go down, the cargo was the most valuable thing on the ship and was to be saved at all costs, even if that meant putting the containers onto lifeboats.
As for the crew of the Enola Gay, it wasn't until Hiroshima was officially cleared as the target (weather had to be perfect) that the crew was told exactly what they were going to unleash upon the world.