Introducing whole milk when?

I am babysitting an 18 month old and her parents have not changed over from formula to whole milk!! I started my two slowly at 11 months and fully by the time they were 12 months, they both liked milk so i never had a problem. This little girls parents say she doesn't like milk.. she had a sip here so far and she did make a face.. but i guess if she only has milk to drink she'll eventually drink it.. What would you do? keep giving the milk or give her water? her parents give her water through the day and formula in morning and before bed.. I have been told you shouldnt give kids too much water cause it just fills them up and they dont get nutrition they need.. My kids drink water only at night time after teeth brushing occures. Help.. i'm stumped.