This is why I don't promise spaces unless I know I will have it (ie. someone is leaving). I have someone who will be going on mat leave and keeping her child in part-time and I was clear with her that I could in no way promise her a spot for the new baby nor a full-time spot for her first child after her mat leave. If it happens, great, but I cannot hold a spot nor promise on a maybe. In your current predicament, I would probably let the six year go, but I am wondering why you promised the space if you were not sure you would have one?