I would absolutely not ACCEPT the family, without meeting the child first. As playfelt mentioned, if they are seriously shopping around they may be doing lots of first meetings to narrow down the field, then bringing the child/ren to some second interviews.
Dependent upon how the phone interview went, I MIGHT be okay with this scenario. The challenge is; with 2 boys in hockey, a daughter in dance, and umpteen other commitments, I don't have time to waste!!! So I would have to feel reeeeealllly good about the person to allow this. If not; I would insist on the whole family.

MammaMia; hope it goes well!!! They are PROBABLY not killers, but better safe than sorry, right!!??