Infants

Hi, my name is Tanya and I am a mother of two children, a girl aged 3 and a baby boy. I was unable to breastfeed either child because I am taking immune suppressant medication due to having had a kidney transplant several years before the children were born. Because of this I gave the children a dummy early on. When I stopped my three year old from sucking a dummy she moved on to sucking her thumb and still does so. I wanted to try and stop it before she goes to nursery and I also worry about the development of her teeth, but I wonder what is the right way to do it and when. Can you help?