I don't believe their deaths are your fault. Sometimes they just die for no apparent reason.

I'm going to suggest from now on when you buy a baby that you use a smaller enclosure that is much easier to heat . Use a plastic medium critter keeper with a UTH for Hermit Crabs placed on the side. Much easier to heat and keep humidity up. Also makes it easier for them to catch their food.

When you are ready to try again please house them this way until they reach 3" then you can upgrade to a larger enclosure.

Im sorry for your loss.