Personally, if I were the landlord, I would be advising the current tenant that they will be liable for the costs of a hotel for the tenants who are to move in, until they have vacated the property.
you can advise that all you like but they aren't liable and its an entirely empty threat, so why bother?
It's not fair on the landlord or the new tenants, but if a tenant with a family is intending to move to a new rental, and that for whatever reason falls through (there is nothing to stop landlords withdrawing or changing their minds), what do you expect them to do? What would you do if it were you? You wouldn't move your children out of their home with nowhere to go, would you?