The Juice Battle

So, I decided around Thanksgiving time that the kiddos were consuming much, too much juice. I have always only given them watered down juice, and prefer white grape to apple for the lower sugar content, but still it seems like there were too many real food battles. It seemed to me that they were full of the empty calories from juice.
Ella has been the worst about breaking-up with her beloved "jew". Wyeth really prefers chocolate milk to juice, and Fara is new to the scene so water is fine with her.
So, really all this has been well and good. We are down to only one or two melting fits a day about how badly Ella NEEDS jew. There has been an increase in water consumption, but I think overall, we have a decrease in total fluid intake.
So.....as I was lying awake at about 3:38 am trying to comfort Fara who is suffering from our 18th illness of the season, when suddenly it occurred to me...what if the lessened fluid intake has compromised the kiddos immune system????
What do you think? I need opinions!