My nephew and niece are picky eaters. My sister in law gives the two of them juice because they won't drink anything else - getting any milk in them is like pounding golf tees underneath their fingernails.
What about chocolate milk? Maybe try that and gradually reducing the amount of chocolate syrup / powder added until they’re conditioned enough for regular milk
Same with juice. Gradually dilute it with more and more water until they’re okay with water
Also I’m not a parent so I have no idea if either of those would really work