I doubt you would be able to use them for milkshakes, as already has been pointed out.
Most of the milkshakes sold as ready made milkshakes are long life milk and what with all the colourings and preservatives, you won't be giving your granddaughter any nutrition atall.
I think redcrx has the best idea with making full fat milk smoothies. Can't you buy the bottled milkshake and pour a smoothie in so that she thinks she is getting the milkshake from the shop?