Hmm, tricky.
Is the childcare issue because she is genuinely upset/feels used, or is it an emotional blackmail thing i.e. if you don't just give me my way like everyone else there will be consequences.
This is important, because if she is really controlling and prepared to be blackmailing, you just have to back off a bit and make sure she doesn't have many levers of control over you, so I agree with duelling, stop using her for childcare. If she is genuinely upset then a different, gentler tack (but one aimed in the same direction as you just standing up to her) might work.
My MIL is hypercritical. However, she does direct it towards herself as much as anyone, so it is really easy to see that she was brought up by hypercritical parents and isn't really aware of it, even though it makes her unhappy too.
When DH and I first bought a house together she really outdid herself in more than one way. So much so that DH did have word with her about it. But it was very much in the "How do you think that made her feel, remember when similar happened to you, how did you feel?". And that really worked.
Also, I am was already the only person of the extended family who engaged with her as her rather than her role. So I took an interest in her hobbies/interests/opinions and spoke her about those, rather than just treating her as a mum/granma/wife with no mind or thoughts of her own.
But she is a genuinely nice person, who was brought up with a lot of criticism herself, and who had some insight into the unhappiness it can cause.
It's not perfect now, but it does work ok and there is genuine fondness between us (have to be careful with that one though, so as not to invoke the wrath of jealous SIL...but that's another story).
So, I would say that if you do go ahead with the family talk, take it as an opportunity to be honest, but be as gentle as you can with it. That the criticism grinds you down, so whilst you don't want to be confrontational, at times you do feel that you have no choice but to defend yourself. Ask if she found criticism from her parents difficult to handle and if you can find a new way of interacting together that makes you both happier.
She doesn't have to compromise as much as you, but she does have to compromise/change a little bit, otherwise you are making a rod for your own back.