Yeeeeesh. Doubt that they will be happy with any multi-child care environment, if that's the case. Was that the only issue? Did you otherwise get a good feeling from them? If so, then this MAY be something you can work with.
I am guessing that they are first time parents, and a little uptight. A good friend of mine used to be like this with her first child. Over time, she eased up considerably. Any chance that you would feel comfortable discussing the issue with mom or dad?
If you got bad vibes in general, I wouldn't even touch this family. Like the gals above said: trust your gut!!!