Personally (and I am sure many will disagree)...I would save the spot for March for family B. You say they seem nice and the child seems nice and finding a good family and child is priceless. The right child and the right family is what makes the home daycare work.
I wouldn't pick another family just based on $$ if I was really happy with the hours and other criteria of Family B.
Plus it only means a month and a half of lost income. I would take it easier those days and prepare more crafts etc and use that time as sort of a breather.
I guess it depends on what you can afford though.
I value the right matching family so much that I do wait months for them and hold spots.
I would get a finance agreement in place though in Jan with Family B and get a non refundable deposit from them. Then in february I would meet with them again to fill out all necessary forms etc.
But that is just me