S.T.
She is definitely dehydrated, and giving her juice to rehydrate her is just fine. Dilute it, though, so she has more flavored water. You should be doing everything you can to get her to drink more.
She might have an ear infection- they tend to drink less when they have ear infections. You might try giving her some tylanol and then see if she'll drink better about half an hour later.
S., mom to 5