Let me cite one piece of literature (among many showing this problem). Dr. Philip Marsh was interested in what happens to the oral biofilm of bacteria developing for children sipping juice or other sweet beverages. Children today are commonly weaned from breast milk or formula to "healthy" juices, typically diluted a bit. This then evolves into juice or other sweet beverages for the toddler in a handy and available sippy/tippy cup.
Using known data on saliva flow and chemistry, food flow, and the various micro environments in and around teeth, he designed a research study to monitor tooth biofilm. Dr. Marsh then introduced juice periodically to mimic a child sipping some juice from a sippy cup or bottle and introduced plaque bacteria (similar to how children get these bacteria in slobbery kisses and feedings with parents). With sampling over time he measured the bacterial balance in the biofilm developing on tooth surfaces.
Tooth biofilm in a healthy mouth typically contains about 1-2 % of the cavity causing bacteria. While mimicking a child's juice sipping pattern Dr Marsh measured the shift in cavity causing bacteria from 1-2 % to over 50%. At that level, those bacteria control the biofilm, supporting the acid loving bacteria and destroying the "healthy" bacteria in the biofilm. Most distressing is that once this biofilm balance has established on the child's developing teeth, about age 2 ½ to 3 years old, it is extremely difficult to change. It will likely be a high cavity inducing biofilm the rest of the child's life.
Ouch! … $ $ $ $ too!
Please don’t introduce juice and other sweet beverages to your child until they are at least 4 years old. Even then, limit it to meals.