If the child is older (3) and you have been giving reminders for awhile then you can likely just start responding to her whining with "you're whining" and walk away ignoring until she uses a normal voice. Often it is just reinforced at home (or wherever) so much that the child doesn't really realize they are doing it. You want to eliminate it which means, acknowledge what the child is doing but give no more attention.

Say "you're whining" then ignore until the child, on their own, repeats in a normal voice then respond happily... "oh, you'd like a cup of water, sure let's go get that for you".

The child should soon enough monitor themselves and lessen the whining because they get no response.