Here are some of the common problems resulting from taking fruit juice instead of water
Stunted growth
Toddlers who fill up on fruit juice may not get the fuel they need to grow. They will have less room for milk and other foods that are richer in calories and other important nutrients, including protein and fat, which should not be restricted in very young children.Diarrhea
Too much fluid can cause loose stools. Some juices, notably apple juice, also contain sorbitol, a natural sugar that can be difficult to digest in large quantities, compounding that effect. Most children who drink a lot of juice experience only loose stools; some develop diarrhea.Diarrhea cause excess loss of water that can very rapidly cause dehydration. This is especially threatening to infants, toddlers, and small children since the amount of water loss necessary to produce dehydration is small in comparison to that of an adult.
Tooth decay
Toddlers sometimes use a bottle more for comfort than nourishment or refreshment. Over-reliance on a bottle, especially to fall asleep can lead to nursing-bottle caries—dental decay resulting from prolonged exposure to sugars, in juice or in milk.Both children and adults should be encouraged to drink water rather than sugared beverages since the presence of sugar is known to slow the absorption of water.
I am sure that you have heard, eat the fruit rather than drink the juice. Kids can see a significant change in their body mass index just be doing this one step. By eating the fruit they will get the nutritional value for much less calories, if there was any true nutritional value in the juice in the first place.