Just tell them like homeschool mom said that you filled the spot based on their end date. If another spot becomes available, you'll let them know.

Who is in your other under 2 spot? Are they almost 2? Do you suspect they may leave for a centre? You could always check in with the other family as well. I always tried to be super transparent with my DC families and as a result they have been with me. I had this situation pop up before and found my other infant was on the list of her work DC and when I asked them they followed up with the DC and found that her name was coming up in month. I didn't want to get rid of them but I am glad I checked as I would have given notice to the other family only to be given notice a couple weeks later by the other family.