My daughter wasn't a thumb sucker, so I don't have any experience with this, but I saw thumb guard that you strap to a child's hand and it covers the thumb to keep them from sucking on it. It's from One Step Ahead. www.onestepahead.com They have a catalog that they send out- you can sign up online. They have some unique products for some of those "mom challenges", plus toys, safety stuff, etc. Hope that helps! I just looked it up in my catalog- boy, it's pricey! $74.95. You may want to try cheaper options first, but if they don't work, at least you know of a back up. Good luck!