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."
Thanks for the pre-formed answer - I like it a lot, and it touches on the two main problems that I am having and it incorporates the fact that I do love their "Suzy". Hopefully, everything comes out "right" the next time my dcm brings up her plans.