Hello, I'm new to the boards but your topic caught my eye. I've been to maui a few times, and in fact last time I stayed in Pa'ia. You may luck out in Pa'ia because there are a couple of healthy/vegetarian/hippyish sort of places, as well as a really nice market to shop for groceries right in town, and it has a great selection of ready to eat foods as well. People are really friendly in Pa'ia, and I'm sure will be very understanding of your needs. I suggest trying out Cafe Mambo, an ecclectic cafe, or perhaps Fresh Mint, a vietnamese place - both are in town on the main street.
Additionally, I recommend you drive over to Makawao to do some shopping at Down to Earth. Makawao is just about a 10 or so minute drive from Pa'ia, and there are signs in Pa'ia town directing you to Makawao. (there is a down to earth down near the airport in kahului as well). Down to Earth is a great little health food store to stock up on a few items.
I am vegetarian as well as gluten intolerant, and restaurants on Maui are very touch and go. Finding vegetarian options was difficult and finding gluten free was much much harder.
The good news is, in such warm weather, you may find that your daughter may gravitate toward things like like bananas, mango, pineapple, coconut, and other yummy raw fruits and veggies - which are NOT hard to come by on maui (and are delicious!)
Oh, and as far as south shore...well, I had even more trouble finding good things to eat down there compared to north shore and upcountry.
My best advice is to get a little cooler and keep it in the car stocked with things your daughter can eat. I did this for both me and my partner (who is vegetarian but not gluten free). I stocked mine with yummy treats, some vietnamese style fresh rolls from Down to Earth, cold drinks, nuts, fruit, chocolate, rice crackers, nori sushi rolls (lots of japanese food on maui - most sushi rolls are ok as long as they dont have soy sauce), tamari rice crackers, dried tropical fruits - dried mango is like candy, etc. It's hard to find good food on the go in maui, especially if you do some of the glorious drives (up the mountain, to Hana, etc). Definitely keep lots of food on hand!
Have fun in maui!!