I've been looking for this quote for ages.
Don't know if this is too bare bones for someone to be able to identify it, but this is all I remember of the quote :(
But you may be conflating two of her traits:
Granny Weatherwax personally disliked young Pewsey. She disliked all small children, which is why she got on with them so well. In Pewsey's case, she felt that no one should be allowed to wander around in just a vest even if they were four years old. And the child had a permanently runny nose and ought to be provided with a handkerchief or, failing that, a cork.
Nanny Ogg, on the other hand, was instant putty in the hands of any grandchild, even one as sticky as Pewsey
"Want sweetie," growled Pewsey, in that curiously deep voice some young children have. "Just in a moment, my duck, I'm talking to the lady," Nanny Ogg fluted. "Want sweetie now." "Bugger off, my precious, Nana's busy right this minute."
Pewsey pulled hard on Nanny Ogg's skirts. "Now sweetie now!"
Granny Weatherwax leaned down until her impressive nose was about level with Pewsey's gushing one. "If you don't go away," she said gravely, "I will personally rip your head off and fill it with snakes." "There!" said Nanny Ogg. "There's lots of poor children in Klatch that'd be grateful for a curse like that."
Pewsey's little face, after a second or two of uncertainty, split into a pumpkin grin. "Funny lady," he said.
There actually several places this comes up in the witches books:
"The trouble is, you see, that if you do know Right from Wrong, you can't choose Wrong. You just can't do it and live. So.. if I was a bad witch I could make Mister Salzella's muscles turn against his bones and break them where he stood... if I was bad. I could do things inside his head, change the shape he thinks he is, and he'd be down on what had been his knees and begging to be turned into a frog... if I was bad. I could leave him with a mind like a scrambled egg, listening to colors and hearing smells...if I was bad. Oh yes." There was another sigh, deeper and more heartfelt. "But I can't do none of that stuff. That wouldn't be Right." — Masquerade
"Granny was an old-fashioned witch. She didn’t do good for people, she did right by them. But Nanny knew that people don’t always appreciate right." — The Sea and Little Fishes,
And some more that I can't find right now.
It is possible that it comes from The Truth, the 25th book in the Discworld series.
In the book, the protagonist William de Worde is handling a newspaper that also employs a vampire named Otto. On page 271 William has just finished his latest story for the newspaper when this conversation happens.
'And it's just a story,' he said, putting away the notebook. It does makes me feel a bit of a vampire-- oh . . . sorry.'
'It is okay,' said Otto. I understand. And I should like to thank you for givink me zis job. It means a lot to me, especially since I can see how nervous you are. Vich is understandable, of course.'
'I'm not nervous! I'm very much at home with other species!' said William hotly. Otto's expression was amicable, but it was also as penetrative as the smile of a vampire can be.
'Yes, I notice how careful you are to be friendly with the dvarfs and you are kind to me, also. It is a big effort vich is very commendable--'
William opened his mouth to protest, and gave up. 'All right, look, it's the way I was brought up, all right? My father was definitely very . . . in favour of humanity, well, ha, not humanity in the sense of. . . I mean, it was more that he was against--'
'Yes, yes, I understand.'
'And that's all there is to it, okay? We can all decide who we're going to be!'
