For myself, there are certain things that I don't like to tell parents on how and what to do. One of those things is age of bottle feeding and formula vs milk. Their child, their preference and in MY opinion, part of the job is to accommodate if possible.

In the past, I had parents send premeasured bottles and I'd just add the water and shake. It really doesn't take me any extra time or work vs getting them a sippy cup ready. I have had premade bottles sent before but found them to be wasted as they had to be dumped after so many hours and if I forgot to send home they'd be wasted. Formula is expensive so I didn't like that as I'd feel guilty! Lol

In the last centre I was at, parents sent formula and water and we mixed.