I would go ahead and interview and say this spot is pending depending on the leave of one of the children and that you will be able to confirm its availability by XX if the new family also is very interested in the spot. That way if you meet a non desirable family you have an out, or if you meet a better one than you can move on swiftly and let this lady know that you need her decision by xx as you have another family in line to start xx if she is unable to continue, and that you need to know by xx.