We've seen a child version of Granny Weatherwax; she's basically Tiffany Aching, solid, serious, and Certain. We've seen a child version of Magrat in Petulia: worried, awkward, earnest. And there are a lot of other young witches of varying temperaments, but none, I think, are quite right to point at as a young Nanny Ogg.
AH, NANNY. Boy, do I know a lot of Nannys. And for what it's worth, and all the changes they go through as they grow up, this is what they're like. If they're not Honey Boo Boo, that is.