Dehydration in toddler

21 month old is sick with low grade fever and had diarrhea twice today. Not terrible. But appetite is obviously low. She’s on the boob pretty frequently throughout the day and night. I’ve been encouraging water drinking. But not drinking as much as I’d like. When I tried to give pedialyte she hates it!! Like literally grabs mouth and spits it out. It’s all a heck no.

But now I’m wondering what’s too dehydrated and how do I keep this kid more hydrated?