For the discussion, I like the idea of stressing that you were always going to change DD once she was in school, because the new place is closer to (actually at?) the school, and its environment is similar to the school. That's more convenient for you and more consistent for DD. Maybe they have increased educational opportunities that Bonnie is not equipped to provide. And, you want to start her now, during the summer, so that it won't be one more big change in the fall, on top of actually starting school. Probably there are other kids in daycare who go to the same school, so maybe DD will make friends with them, and thus have some friends at school once it starts.
I think if you stay cheerful and matter-of-fact, she would have to be very bold to outright bring up being "fired" herself. You could assure her that you hope she and DD will continue to have a relationship through occasional extra babysitting opportunities, if in fact this is true.
And, Lynda_34 makes a good point. It really depends on how you think Bonnie will act after you've given her the 30 days' notice. Certainly you would hope she would continue to be kind, or at least professional; but it might be wise to have a back-up plan, just in case, which you might not need at all, or only for two weeks of four, etc.. Depends on what's feasible for your family--having a relative in for a week to watch DD at home, taking a week's vacation yourself, paying Bonnie AND the new place for a week, paying Bonnie AND a local teen/college student for a week, etc..