Hi, Normally paed will only prescribe ORS when LO having diarrhea, to increase body's salt. For normal hot day, you can always ask your daughter to drink more plain water, or you can make more juices for her consumption. This will maintain the hydration in her body.
Mummy, ideally not to give hydration salts to bb. Water and milk is still the best source of hydration. You can include soup for bb too.