You both sound like you respond to vulnerable feelings by feeling defensive and reacting bitchily (even if you're only saying it in your head at this point).
You can't change the way she responds to her feelings but you can change the way you respond to yours. You can accept that her feelings are, at least in part valid, but also quite possibly a defense against her own sadness about not having kids, while not taking it as a personal attack on you.
"phew that looked hard, glad I'm not wasting hours of my evening like that on the regular hahahahah"
"Yep, it was pretty exhausting tonight! Thank god most nights are nowhere near this bad."
"Perks of being a mum eh"
"Haha I know, probably payback for the messes I made when I was a kid"
"I think I value my sleep too much to have kids"
"Ah, it's not such a bad swap though a couple of extra hours would be nice"
"feel so bad for parents, it effectively means you don't get to eat in peace for years on end!"
"All part of the job I guess!"
"eugh the joys of being loved by a toddler hahahah"
"Yeah, good thing it's the love that sticks and the dirt that washes out" or even just "Hard to mind really when he's so sweet"
"yuck, don't think I could bring myself to clean up anyone else's sick. Makes me vomit just watching this. You're so brave!"
"Oh thanks, I feel a bit vommy myself. You're right, this is the worst bit!"