The way I see it, no one no matter what age wants to eat something pre chosen for them, you want to choose your own food to put in your own body, and children are no different.
By including your child in the food shopping for their packed school lunch they are part of the process and mentally in control of their own food choice.
Create food storage sections which has multiple options, all off which you are happy with as a parent and are healthy and appropriate for lunch. Allow your child each morning to pick their own fruit or snack, this will give them independence and means. they are more likely to eat their lunch as they picked it themselves that day.