When ever I take on a part timer like that it is made very clear to them that it is not permanent and that is subject to a full timer coming along. The PT is then given two options - pay the full time fee or give notice. Just tell her that you will fill in till April so she will have care till she can find someone else and that you are going to continue to advertise for a FT since you realize the finances are important. Do not let on that you have had contacts as it is non of her business. Then go ahead and interview, etc but as much as possible honour the April timeframe. We try to go out of our way to be nice to parents even when it hurts us financially or personally but if the situtation was reversed parents rarely give us the sme consideration.