Well, it does not matter whether or not it was your fault, that is neither here not there. But it is a problem, and needs to be addressed.
I wonder if difficult child 2 is feeling nostalgic about the crib? Now that he has been back in it, he remembers being there as a baby and wants to continue being there? Anyways, something has to be done to keep him out so he does not hurt the one year old, especially now that he has discovered aggression.
Perhaps move the crib into your room?
