I've heard this letting go of/taking on of responsibility compared to letting go of a compressed spring. If you let go of it suddenly, the spring will fly out of your hand in any direction. That would be rather like the situation you would have if you wait until your kids are grown, ready to make their own decisions on their own. But if you release the tension on the spring very, very slowly, it will just lay there in your hand at the end. So try letting go of a little responsibility by sharing it, maybe. Say, "If you think you 've maybe outgrown your gluten sensitivity, or you think it won't bother you as much now, don't try it on your own, or sneak it. Come to me, and we'll try it together." And then let her try a little, and the two of you together monitor whatever reactions she gets to it, so that you can help her come to the right decision, that it just wasn't worth it.
Mind you, I've never had a child with any kind of food sensitivity, so feel free to tell me I don't know what I'm talking about!
Hugs, and sympathies,