I have had this problem but his year my second grader announced that she always had the healthiest lunches at school and she liked that because she was different. That alone made me glad I stood my ground.
I send treats randomly here and there throughout the month. I try to change the days and what I am sending to make it a surprise. I do send homemade items though which is out for you.
There are many healthier options of pre packaged such as fruit leathers, my kids love the dried fruit that is shaped like bug, sun rype I think makes them.
I also stick chocolate chips inside raspberries, they love this and on occasion I dip a few pieces of their fruit in chocolate.
I also purchased pre packaged granola bars every once and a while too just for an added treat. They aren't the greatest but they have less sugar than a yogurt tube so they aren't awful either