6 year old and baby bottle. help!

I'm new to this forum so apologies if I have posted in the wrong section.

I have a 6-year-old daughter with autism. She is non-verbal, not toilet trained and suffers anxiety. 

Among the million worries I have for her is her addiction to her baby bottle. She will only drink milk and only from her baby bottle.

We have tried:

cold turkey x2- both times she ended up in hospital with dehydration.

Watering her milk down gradually.

Milk in cup and water in the bottle.

Every style of cup/beaker possible.

We have given her sensory chews.

Her teeth have suffered for this and the school (inclusion unit) refuse her the bottle in school, which I understand but with the warmer weather, it's worrying.

We are seeking help from a psychologist but in the meantime any advise would be hugely appreciated.