I think you should definately contact this parent. If you didn't put any commitment in writing, then she won't understand why you are upset and thinking of filling that spot without her. In a verbal agreement, there is so much room for interpretation and "mis-remembering." I think the others are right, she probably feels she has no obligation to you and is looking for something better for her. If you value the 'upfront' approach from parents, you should contact her, tell her you saw the ad and ask her what she is planning to do. You were holding the space and was expecting her to come but you didn't know the days/hours she needed not whether or not she was coming at all.