I keep my thermostat set at 64 during the day and 56 at night and on weekends (and any other time I can get away with it). I do childcare 5 days a week in my home, but tell the parents to bring slippers for their kiddos. I also have plenty of extra sweaters for them if they seem cold, but they are running all day and usually end up sweaty anyway.
At night I have a thin cotton thermal blanket under a huge wool stuffed comforter and a warm blooded toddler to snuggle up with, so we stay super toasty at night. In fact, I still haven't taken my air conditioner out of my bedroom window yet, so we effectively still have the bedroom window open in there, so I am sure it is getting down into the low 40's on some nights and we are snug as bugs in rugs.

Oh, I don't heat my upstairs either, so the open window in the bedroom is not really affecting the heating of the downstairs either. (although I have to get that dang thing out of the window this weekend for sure...)