Trans fats: This is one bad fat that deserves to be on the zero-tolerance list. "Just be careful not to get so hung up on choosing foods that are trans-fat -- free that you forget to pay attention to how much saturated fat and calories they contain," says Taub-Dix. Both types of fat contribute to heart disease and other health problems, and there are many foods with no trans fats that are loaded with cals.
High-fructose corn syrup: Treat it as you would any other added sweetener. "There's really no difference between high-fructose corn syrup and regular sugar in terms of how it can affect your child's weight and health," says Taub-Dix. Just do your best to keep overall intake low because kids are generally oversugared anyway.
Food dyes: Your instincts are right on. "A growing body of research has found an association between artificial colorings -- ones with numbers, such as Yellow 5, Red 40, Blue 1, and Green 3 -- and behavior problems like hyperactivity," says Michael Jacobson, Ph.D., executive director of the Center for Science in the Public Interest. The dyes, found in everything from cake mixes and yogurts to breakfast cereals, are being phased out in Europe, but consumption in the U.S. is on the upswing -- it's increased fivefold over the past 30 years. "We don't know exactly why they have this effect, but they can have a big impact on some kids' behavior," adds Jacobson. "It's smart to choose foods without them when you can."