The reason it is hard to stop thumb sucking with all the negative tools like bitter polish is because thumb sucking is such a comforting thing to a child and they will not want to give it up when you put bitter polish (which has chemicals in it) or shameful thumb guards on. Try the positive approach…there is a fun thing for children called “Thumbuddy To Love” and it comes with a thumb puppet/story book with special tips on how to use it. Teaches kids in a fun and positive way to let go of something so comforting. Worked for my kids. Here is the info: You can get it on Amazon or thumbuddytolove dot com.
The sooner the better because prolonged thumb sucking can lead to buck teeth.