My friend Carrie has a son and daughter in grade school and three-month-old triplets. When they go out in public, they turn heads, and uncomfortable as that may be to a family of (mostly) introverts, it's nothing compared to the rude and unthinking things people say. When they went as a family to their polling place yesterday, one woman told Carrie she spoke for every woman in the room when she offered her condolences. Condolences! On a passel of healthy children! And we're not talking about a whisper to Carrie alone -- no, we're talking about loud enough that not only the children could hear, but others around could take note of how the stranger assumes their mother wishes they'd never been born. On another occasion, a woman told her she'd have killed herself if she found out she was having triplets.
I'm sure it makes quite an impression, the young mother dealt three of a kind in five card draw. I understand the impulse to reach out, to offer support and sympathy for the sort of extreme parenting most of us never have to contemplate. But finish the thought, people! It is possible to offer support and even sympathy without implying -- let alone stating outright -- to the children that they're an unduly onerous burden.
May I suggest "God bless you," a gold standard among things to say to mothers of babies for a very good reason: it conveys all of "you poor thing" while also bestowing an actual blessing. Or, if you're an orthodox atheist or have some other reason to resist invoking even the most metaphorical almighty, you could just tell her they're beautiful and she's clearly doing a great job, which is always true, even if it isn't.