The foundation of a healthy lunch is that they have sustenance that will keep them full and energized for the rest of the day; for sustenance, the best thing to do is pack your child a sandwich.
Even though they may not actually eat it, it is also good to pack them fruit; as important as fruit is for adults, it is even more important for children, so make an effort to find a fruit that they like – or, to find a dried fruit that they like – and make sure they have this in their lunch when they go to school.
Packing your children snacks that they can munch on throughout the day will also help to keep them healthy; a couple great ideas in this department are Cheerios and granola, which you can pack in a plastic baggie for them to pull out whenever they need a couple handfuls of snack.
And considering that the majority of children at your child's school probably do not have healthy lunches, you may also want to give your child some form of desert, so keep your eyes out for “healthy” desserts, and you will be able to keep your child feeling like they are not missing out!