Do you demand feed? If so, just follow your DDs lead. She will let you know if she gets thirsty.

Personally, I would probably offer her a little water every now and then just to be on the safe side but the formula should be enough to give her a drink & feed.

I would also just let her lie around either naked or with a nappy to help keep cool and wipe her with a damp, cool (not cold) facewasher every now and then for a bit of relief.