From my personal experience as a single parent, the welfare office will insist that 2 rooms are available (one for parent, one for child).
Not sure about the sharing with landlord aspect but I would imagine it wouldn't be straightforward and they would be likely to investigate, particularly if you are of opposite sex to the person renting from you.
From a community welfare officers point of view, a woman claiming rent allowance for one room(which she intends to share with her child) in a house she shares with a male landlord........You can see why it may arouse suspicion.