There really is not a lot you can do to convince her that it is a bad habit as it is pretty impossible to reason with a three-and-a-half-year-old. From what you describe it sounds like she tends to suck her thumb when she is bored or tired. Most children this age are using their thumb sucking as a comfort mechanism. In terms of doing something about it, it depends on how much it bothers you. Many pediatricians and psychologists would suggest not doing anything and give her a little bit more time to decide on her own that she doesn't need to suck her thumb. In terms of the mechanical consequences, dentists do tend to be concerned about what it might do to the teeth and it is true that children who suck their thumb a lot can cause some problems with their bite because of the pressure the thumb exerts on the front teeth. This tends to be more of an issue in kids who suck their thumb frequently, and not as much for the kids who only suck their thumb when falling asleep. Thus it is recommended that children not suck their thumb after about age five when this becomes a bigger concern.

In terms of what you can do about this there are a number of possibilities. One thing is to limit the time when you know she likes to suck her thumb, in other words have her not have too many times when she is bored. Find other activities for her to do when she is traveling in the car such as games and other things where she needs to be able to talk and use her hands so that she can't suck her thumb. It would also be reasonable to limit her television time. In general it is a good idea to limit television time to a maximum of an hour a day. You can also just try and make it inconvenient for her to suck her thumb. For example, whenever she is sucking her thumb have that be the time where you want her to participate in some activity or fun thing that she wants to do.

There are substances that you can get in the drug store that are bitter tasting that you can paint on the thumb to dissuade children from sucking their thumb. Whenever they put the thumb in their mouth and they taste the bitter taste, they are then motivated to take the thumb out of their mouth. If your child does not want to stop sucking her thumb then I do not think this is a reasonable thing to do at this point. This type of treatment is more useful in slightly older children who actually want to stop sucking their thumb but need help doing it.

Another thing that some people have tried that has had some success is to actually tell the child that she has to suck her thumb for a period of time. In other words you pick a 15 minute period of the day when she has to go suck her thumb and in that 15 minutes every day she needs to stop what she is doing, go sit down, and suck her thumb. In choosing the time of day you would want to make it as inconvenient as possible, when she is awake and wants to be playing. Again, this is something that would probably be more successful in an older child who has some motivation to stop sucking his thumb rather than a child the age of yours.

Probably the best thing that you can do at this point is to try and get your husband and his family to back off a little bit and not become so upset by it. There are very few children who continue to suck their thumb into adulthood and you have to decide if the controversy and upheaval you cause by trying to force her to stop are worth the price of just letting her suck her thumb and get over it at her own pace.