It's important for your kids to try new foods at home long before you put it them in their lunch boxes -- if you surprise them at school with a new food choice, they'll most likely toss it without even trying it! Make sure that your child is familiar with the foods you're packing before you pack them. Help them develop a taste for healthier foods at home -- then they will be more likely to eat those foods rather than "trade" them for something they find at school.
Kids learn best by example. Eat a variety of healthy foods at home and take your kids grocery shopping while teaching them about fresh fruits and vegetables. Teach them how these foods are grown and where they come from. Engage them in the process of choosing their own healthy lunch snacks.
Want to compete with quick and easy junk food? The secret is to make healthy foods just as quick and easy as junk food. Plan different healthy lunch options in advance, when you have more time to think. Make bulk meals on Sundays that will last throughout the week. You can even have your child help you cook the dish on Sunday night before the week gets busy. Discard any choices that take too much time to prepare and keep the ones that you can do quickly without stress.
Read labels -- but don't worry (too much) about calories, fats, and sugars. Sure, these are important, but if you start adding more natural choices and less processed foods, then you're already ahead of the game. Make gradual but steady changes -- this way, your child is more likely to accept them.
Collectively, most public schools are making a better effort to provide healthier choices for their students, but don't take this at face value. Schools usually have a website that provides nutritional information on daily lunch options. If your school doesn't offer this feature, insist they create one. Become familiar with the cafeteria food options and don't be afraid to speak up if you disagree. Several schools have responded to parents' concerns about healthier food choices. Ask for a salad bar with fresh fruits and vegetables. Several schools have installed these at the request of parents and students, with excellent results.