I would guess that it’s grammatically necessary so that “וְאֶת בֵּיתָם וְאֶת זַרְעָם וְאֶת …” attached to the host & hostess rather than to their “בית”. The phrase would not be ambiguous without the “אותם” (The Muppets’ talking houses notwithstanding ☺) but it would be grammatically incorrect.
Edit: In this post’s comments, Alex suggests a different possible grammatical issue. I’m far from skilled enough in dikduk to guess which (if either) is correct, but my suggestion remains the same: “אותם” is grammatically necessary even though there is no semantic confusion possible.
(There are plenty of examples of this in English; e.g., the meaning of “I no like!” is perfectly clear even though it’s grammatically incorrect.)