A balanced lunch diet is essential as you prepare the meal.  Obviously finger/hand foods are preferred -- it becomes less convenient when your child needs a fork or spoon to eat, although the occasional pudding with a plastic spoon can be an option. And have the child help prepare the lunch -- this way there are no surprises which sometimes backfire, and the child often anticipates and looks forward to the lunch as the time approaches during the day.

Generally lunch has carbohydrates for energy -- so starch in form of bread and maybe crackers can be provided.  Protein is typically in form of cheese, meats/fish (tuna), soy/tofu, egg or even peanut butter.  Thus sandwiches are a relatively easy and healthy way to provide these nutrients.  Sandwiches can start with various types of breads (including wheat, rolls, pita pockets, or wraps), and mustard added as a spread (avoiding the high fat mayo or dressings).  Other options can include hummus, cheese sticks, even salads, sushi, or pasta (but may need fork).  

Vegetables -- carrot sticks, cherry tomatoes, slices of red/yellow/orange peppers, pickles -- and fruits -- apples (sliced), grapes, berries, orange or mandarin segments, or bananas are popular.  

An alternative is to give crackers and veggies with a spread such as hummus that occasionally replaces the sandwich.

Drinks should be water from the fountain, milk (nonfat), or 100% juice limited to 4-6 oz.

Note that chips, cookies, etc are not included above.  These can also be added for energy but should be limited in quantity so other foods are eaten.