I generally paint in oils and acrylics, but know a little bit about watercolors.
Dick Blick is a reliable online retailer with a large selection and good prices (there are several brick and mortar stores a short distance from me, but I have shopped them online as well).
I use
M. Graham artists' watercolors. M. Graham paints are high quality and reasonably priced (Winsor & Newton is extremely popular, though a bit more expensive). Always stay away from "student" watercolors (as opposed to the more expensive "artists'" grade). Artists' grade paints have more pigment, purer color, and are easier to work with. Blick also makes their own house line of paints, which I'm sure are of decent/good quality and would save you a few dollars.
I find it best to stick with one brand (particularly when it comes to watercolors, less so with oils/acrylics). If you know what she has (and she likes it), stick with that. "Cadmium red" (for example) can be found in every line, but it can vary considerably. And not all lines mix well with each other.
If you want advice about what colors to buy her, let me know. With a few good colors, you can mix an infinite number of others.
Hope this helps. Nice of you to encourage your granddaughter's talent.