I think you just have to have an honest dialogue.
Next time the mat leave comes up say "I have been thinking about what you said about maybe keeping her in part time. I thought about it the last few weeks and I feel awful for saying this but I don't think I can do it. I really am looking for full time families right now and going to part time would affect our days and my finances to be honest. I love you guys, love little Susie but I also have to think of the finances and logistics of it. I am not able to do part time."