According to some researcher with grant money, we should investigate how oral bacteria chemically signal to each other, so that we can disrupt that and keep them from aggregating and taking over an area of the mouth.
I have a cheaper and therefore better idea -- don't give your kid fruit juice, soda, vanilla wafers, or other high-carb foods. The dentist-turned-anthropologist Weston A. Price figured this out decades ago, and all of the archaeological evidence since then confirms it: hunter-gatherers don't get cavities, while those who adopted agriculture immediately suffered from rotten teeth. (See some of his pictures here.) The difference is how much digestible carbs you're taking in.
Fruit juice is just a bunch of sugar dissolved in water, along with a few nutrients that you can easily get elsewhere. The most obvious alternatives are real pieces of fruit. If you feed your kid half of a medium apple, that's only 10.5 g of net carbs, with 9.5 g of those being sugars. In contrast, your kid can suck down 1 cup of apple juice no problem. Even the unsweetened kind has 28 g of net carbs, with 24 g being sugars. Not very far from a bag of Skittles.
If you're worried about vitamin C, cutting down on carbs automatically helps, given that higher glucose levels will displace vitamin C from accessing its transporter. If that still doesn't work, cook some liver and mix it in with something they'll find more palatable. Oysters work too. If they don't like the taste, tough shit -- they'll thank you in their 20s and 30s when their skin won't be destroyed by chronic sugar overdosing.