Get a note from your doctor that says something like "Based on the dietary needs of this family and the high probability of issues being genetic, this child needs to stay on a gluten/wheat/barley free diet." See if you can get the doctor to include something about why testing would not be beneficial at this time (going on SAD for 30 days, then the testing, at a crucial brain development time or some such.)
You may need to provide the food yourself, which would also include snacks. Which depending on your state and the child care laws may or may not be allowed. With a doctor's note they should HAVE to provide the food that she needs if the state says that no outside food is allowed. (Laws governing day cares and preschools are just outrageous in some states.) If they have to provide, do expect it to be more junky SAD without gluten food.