Selective eating is not the same as picky eating. Have a read through this article if you don't know what I'm talking about (thanks to my friend Kathy for sharing that).
My son Cade has a variety of special needs including autism and ADHD. Lesser-known to many is that autism very often comes with eating and digestive problems. In Cade's case, blood tests had revealed allergies to gluten and dairy, but not the typical allergies. Mostly when you hear allergies doctors are thinking of IgE allergies, associated with a histamine response. Cade has IgG allergies, which are associated with behaviors.
Now, not only did we have allergy issues, we had a very selective eater. Cade's diet largely consisted of Cheerios, Rice Krispies with milk, warm milk, fruit juice, and graham crackers. We had had him on a variety of toddler "tv dinner" style meals, which were eventually discontinued, and he didn't like any of the successor varieties, which meant his veggie supply vanished overnight.
Eating therapy was a complete bust. Attempts to force him to eat were met with tantrums, spitting out food, and other traumatic behavior. Guidance to starve him into submission were misguided; kids like Cade will indeed starve themselves before they will eat new foods.
Cade's breakthroughs came around the age of 7 1/2. A DAN (that stands for Defeat Autism Now) doctor recommended we increase Cade's zinc intake. Zinc is quickly depleted with all the sugary foods Cade was eating, as it's used to produce insulin, and both zinc and B-12 are important in maintaining an appropriate sense of taste, as described here. In our case, we put Alive! liquid vitamins in with Cade's juice. He had a lot of flexibility here so we ended up putting a lot of supplements in there. The DAN doctor kept loading on more and more supplements.
Here I would caution parents. Very little else that the DAN doctor said to try really seemed to work out. The GFCF diet and zinc were the big winners for us though, but eventually, between lack of further progress, differences of opinion, trauma in and after doctor visits, and changes in coverage, we gave up on this doctor.
Zinc seemed to gradually open Cade up (in the space of a couple months I would say) to new foods, but it was still not at all easy. First we tried the slow introduction of new foods. For example, we'd try putting peanut butter on graham crackers, or other crackers. This utterly failed. However, we were easily able to substitute gluten-free graham crackers for regular ones. This worked brilliantly, and demonstrated at least a little flexibility in Cade's palate. We also substituted almond milk, rice milk, and flax milk for regular milk, and introduced him to So Delicious. These also worked and got him off dairy. At the time gluten free Rice Krispies had just come out, so we swapped him over to those. (Now they have been discontinued, but Cheerios are now gluten free, so we reintroduced Cade to those and he eats them like crazy.)
From there I believe, although the "slow introduction" of new foods was not workable, we were able to branch out, using what he already liked as a basis. So, since he liked graham crackers so much, we gave him varieties of gluten-free cookies (either homemade or store bought). His mom gave him French Fries--another bland food, but still slightly different taste and texture. His mom also gave him other varieties of cereal (going from his Rice Krispies to Chex and Crispix for example). Even small steps like substituting similar brands or slightly different varieties of foods from the same brand can be important steps. We eventually stepped Cade up into occasional pancakes. I even got him to eat omelettes sometimes, though I put maple syrup into them so they were sweet. After awhile of this we somehow got him to begin eating things like meat and veggies, even standalone, though he still usually insists on eating one entire food group at a time if they are discrete to begin with. And his stepmom cuts veggies extra small in foods like chili and pasta sauces to keep him from picking through.
If you have a selective eater in your family, be understanding of their problems. Understand how traumatic new foods can be to selective eaters, and that eating therapy may be useless. Try to educate yourself and your family to understand and appreciate the depth of the issue. And remember that mostly what you can do is bring the horse to water--again and again. You can't make them drink (or eat). Maybe eventually they'll open up as Cade did, but maybe they won't. Keep trying, but don't fall into the trap of thinking that parents who can't make such progress occur are bad parents.
Anyway, Happy Thanksgiving! If you have a selective eater, do your best to survive this holiday. I know it isn't always easy.