It doesn't hurt the milk as long as you don't microwave or boil it. Some babies are fine with milk at any temperature, I do have my daycare warm up my DD's bottles as she prefers it and they have a little crock pot thing that they warm them up with, not sure if it is technically a bottle warmer as I don't know what they look like. I always just warmed her bottles at home when I wanted to by running under a hot tap or putting them in a pan of hot water for a bit.
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