Ya - if they are preferring sipper cups as their choice of fluid intake I would offer them milk in the sipper at one of your meal times whichever works best .... I generally offer milk at main meal time and water at snack times unless we are having cereal for morning snack and than I offer milk at that time frame and water with lunch and pm snack.

I too have the majority of my crew choosing water over milk even when offered it and I am not concerned in 'making' them drink milk - they get lots of calcium rich foods and we spend at least two hours a day outside for their Vitamin D intake!

Honestly human beings are the ONLY mammal that consumes the 'breast milk' of other animals ... you have to wonder WHY that is and if it is really 'necessary' to our dietary survival cause in reality we all evolved from areas where 'cows / goats' were not predominately available for milking and therefore 'milk and dairy' was not available to the entire human population and so forth!