Ignore whining. Say "I can only listen to your big girl/ boy voice". If child is having trouble re toning his request then model it for your child. Say the request for them to repeat. For example... "I want a drink please, mommy!" ( in a normal/ nice tone).
Also you can record your child's voice and play it for them. You could imitate them to show them how they sound. Another option - you can say one phrase in two tones and have child identify which sounds better. If they pick the whiny one then just use a whiny voice when talking to them all day and see if they call you on it!