M.B.
There's a place right in Hartsdale that looks really great - but they don't do part-time. But I think they have a great facility and have a great methodology. Very much like a school. It is called Creative Beginnings Children's Center. I'd get in touch with them soon because they may be filling up.
There is also a new one opening in Scarsdale - Cornerstone Children's Center. But it is supposed to be open next Monday and doesn't look like it is quite ready. But they are going to have part-time - not sure if that means half-days or shortened week.
There are a bunch of different locations for Children's Corner in Westchester. My daughter went there for a year which was fine but not great (not so structured, not so professional, very dependent on timeouts). It is possible that some locations are better than others (certainly true with respect to space & facilities). Worth visiting and hearing about their programs.
Hope that helps - good luck!
I hope that helps!