I would e-mail back that you are bringing a portable crib. Then bring it. There is no lying there. You aren't saying that you will use it, but you are letting them infer that.
Then if they still have a problem with 3 individuals in a room meant for 2, they can say so.
we stayed in a B&B when ds was about 2 and the owner was happy to have us there. so i would agree that there are some that probably don't want children i know there are plenty that are also family friendly. i could definitely see them just being worried about the fire hazard issue.