I think you're going to run into that "If we make an exception for one person...we'll have to make an exception for everyone" dilemma. Even if you try to convince them that you will bake a low-fat, applesauce instead of oil, fruit based MUFFIN with little or no icing on top (yes I know a muffin has as much/even more sugar than cake but I do think it helps if some of the ingredients have at least SOME nutritional value)...how will they explain that to everyone else to whom they've told "no"?
My daughter's preschool has the same restrictions; the most they will let parents send in is 100% fruit juice ice pops. Woo. I am thinking of getting some cute molds and making my own 100% fruit puree pops just so I can satisfy my creative urges. Hey they didn't say they have to be from the store
! I do have completely mixed feelings about the policy; if they allowed treats, I personally would bring in "healthier" treats but this year my DD went to a different summer preschool with no food restrictions, and I have to admit it was really beginning to bother me that someone brought cupcakes as a "snack" literally once a week. I was like, seriously people my child does not need to eat cupcakes this often. And of course they were straight from the supermarket too
. I think schools do it because otherwise parents have no say about their kids' consumption of junk food.
I know allergies factor in too but what you're discussing isn't really about allergies, it's about "healthful snacks."
Try to find something else clever to decorate. For a Thanksgiving party at her school last year, I signed up for turkey sandwiches. I cut the bread with a turkey-shaped cookie cutter and then colored some cream cheese and piped it on the outside
of the sandwiches sort of in the style of the turkey cookies here on CC. The kids totally went crazy for them. How awesome is that, kids going crazy for a sandwich. Maybe you can come up with some other clever, special healtful treat-snacks.