Oh, I'm so sorry. You did the best you could, and their death is not your fault.
Don't beat yourself up about it. It is very, very hard to raise little ones by hand and really success is 20%. I've never raised rats, but I've raised bunnies, loosing as many as I save, and rodents just don't thrive without mom.
They were loved for their little lives and thats what matters. *hugs*