OK--the way I see this, you have two separate issues going on...am I right?
First the O. day per weeker that is supposed to "pay" with play dates that are NOT happening every month, as agreed?
Upon her next pickup, say to the mom "Is this Saturday at 10 good for the monthly play date? I have a few errands I need to do and could really be more efficient with Sammy at your place." Stop. Let her respond. It will all become clear to her at that very moment.
Second, the 2-day per weeker, that gets picked up late and lingers with mom even later that also uses you for extra "free" care on days when the preschool is closed. Get a copy of the school calendar. Mark the "closed days" that you WILL be able to cover & watch Janie for those days...for an additional $20 (or whatever you charge full day). This woman also needs to be given a set pick-up time. have all of her kids "stuff" packed & ready to go by the door at that time. If she presses you about it, tell her that the 7-8:00 pick ups are making your dinner/family time too hectic and you need to set a more rigid schedule with your family.
I know it's awkward to bring up the subject of money with people you know, but you are not Salvation army! You're a childcare provider! It amazes me that people would take advantage like that....but it seems like a lot do! Really--these people are going to look to YOU to set the tone, policy, rules, etc. so....DO IT!!! Good luck!