Sadly, a lot of people are (in my opinion) mistaken into thinking kids NEED an exorbitant amount of milk to be healthy. If only they realized the amount of sugar their child is ingesting.
I don't offer any milk to drink (during or outside of childcare hours). We have milk in cereal, in recipes and we have other sources of dairy (plain greek yogurt, cheese etc). We are outside daily for long periods for vitamin D. There is no need for milk. All it does is fill little tummies making them not NEED to eat food that they think looks suspicious.
Odds are you will not be able to convince them that you provide enough. All you can do is continue to repeat what you have noted. Milk is provided, they can request more if they so choose. At 3.5yrs of age a child is old enough to know if they want more. You could send home resources about other sources of calcium, vitamin D and what ever else they think milk is magically providing.