I'm also the mom of a thumb sucker, and I was too - until Kindergarten. My advice - don't worry & be patient. We tried many things to prevent my son from sucking his thumb, none of which worked. The same thing with my own mom - nothing worked until I decided for myself, that it was time to quit. School is a wonderful way of working things out, that parents can't. Peer pressure is a powerful motivator, and the first time I heard thumb suckers being made fun of at school (even though I only did it at night, and never in public), that was it for me. For my 5yr old son, he also knew it wasn't something he would ever tell his friends about, and began to feel it wasn't what "big boys" did. We helped him reach his decision to quit, by giving him an incentive. He was able to get a new pet (he chose hermit crabs) once he'd gone a month without sucking his thumb. It was a piece of cake for all of us, b/c he was old enough to reason with. I know you may be worried about teeth. My son's teeth all came in very early, and our dentist reassured us that if he quit at least before he was well into school, we should'nt worry about any long-term damage. (His teeth are beautiful and straight now, two years later.)
I must also say that some kids just have an oral fixation. Since conquering the thumb-sucking, my son has overcome nail biting and lip chewing. Kids are hard to keep up with, aren't they? :) Good luck and hang in there. At least our kids never give us a chance to be bored!
Sincerely,
M. B