I would transition them to cups - if they use one for water easy to dump it out and use it for milk too so they get to see that "all" liquids come that way. Many parents continue to send bottles of milk just because it is easier to measure out and transport - especially if bringing formula or homo milk since I provide only 2% in a cup so all bottles to be taken as bottles must be sent or any milk other than 2% must be sent even if I serve it in a cup.