I don't think there is anything wrong with drinking a lot of liquids if they are nutritious. Our daughter drinks a lot more than most kids her age, and her father drinks a ton of water. I think there is some constitutional thing there, although I don't know what that is.
My questions would be, is your granddaughter getting milk as well as rice milk? (You didn't mention any dairy intolerance). Is her diet healthy overall? Is she getting exercise? Does she show any signs of diabetes or any other health problems? Is she overweight? If you're not sure about these things, I would ask the pediatrician.
A lot of people post questions about whether to call the doctor about this or that, and in my view, if you're asking the question, then you should call. Even if you speak with the doc or a nurse over the phone, you'll learn something, and an appointment may be warranted. That's part of their job as healthcare providers, to guide you when you don't know something about your child's health. There's never a reason NOT to ask the doctor things if you are concerned - better to be informed than in the dark.