Thank you guys for the support and all the good advice. I should provide a little more background.
First, I actually have been communicating with the moms already. I've had to in order to plan playdates and whatnot. It was a little awkward at first, but not too bad as I have always been involved with stuff so I already know these moms...they just normally have dealt with W. Even the mom that was emailing W, i have been chatting with her all week trying to set up a sleepover for D's bday. So, got you on that one. In this case, even though the mom had been planning a separate event with me for the following day, it was W who she copied when planning the sleepover at her house.
As far as the kids and their expectations, they actually do really well with that. They see how their M does not include me on all the plans and how it backfires a little every once in a while because of it. Nothing major, just them seeing that I am out of the loop on something that I should be included on. The other day W said something to D about plans and Ds response was "you better call daddy and tell him about that". I have never told her to do anything like that, but she came to me and told me that evening that she said that to W. W did call me during the day and tell me about the event in question, but I didn't know at the time it was because D told her to. So, the kids are in the know and on their own accord are trying to keep W respectful of me. Love those little guys.
I do realize that I am being too polite and not direct enough. 2x4 accepted. That actually isn't hard to hear so that's barely even a 2x4...more like a wrist slap I'll keep working on that.
Kids do have phones so I'm sure that D probably already knew about the sleepover. It isn't really even about whether or not she already knew, its about W not interfering with the planning of things on my days. Again, point taken about being direct with the boundaries. Going forward I'll work on being more clear with my boundaries and consequences...and be ready to enforce them if broken.
I do realize that this was not too big of a deal. W got an email and forwarded it to me when she got it without approving any plans with the other moms. It just irked me that she talked to D about it before mentioning it to me. It has the possibility to set up expectations that I might have to break if it conflicts with existing scheduling.
So I did not respond immediately because I recognized that I may have been feeling overly emotional about something that just wasn't that important. Not that boundaries aren't important, but there are a lot of battles to choose in this journey and some of them just aren't important enough to care about. I wanted to wait and see how important this one was to me before responding. Turns out, it wasn't really that important to me once I calmed down.
What I decided was that no reply to W was necessary. I told her in my initial response what I wanted, which was for to run potential plans by me before sharing with D. She will either do that or she won't, I don't feel the need to explain myself. If she does something else in the future along these lines and I don't like it, then I can practice being more direct like you guys counselled me here. At this point I realize it just isn't a big enough deal to waste any more head space on.
I'll keep working on that.