I had todo something similar with my mom-different situation but it was a "lay it all out there" kinda email. It worked for awhile but then she reverted. But my mom is a classic narcissist so there's no helping her. It did however help me. I laid EVERYTHING out there. And now she knows. Now she has no recourse when she does something I've expressly told her not to do and I stop letting her see the kids.
That bein said congrats on your newbie! I can not imagine 6 kids! I'm done after 3. Good for you! And just let your mom-and everyone-know that negativity will not be tolerated. If its THAt hurtful then do something drastic to get your point across. Whether that's blowing up or just passively ignoring them for a week-whatever. Don't indulge their negativity.
Hope it gets better!