Fluoride when taken in internally during tooth formation, it becomes incorporated into the enamel , making the tooth less prone to decay. Once a tooth has been formed, fluoride take internally no longer has any effect on its structure. External treatments such as fluoridated toothpastes and rinses, do have an effect. Fluoride will often stop the progression or early decay and has been shown to prevent the formation of cavities on the roots of the teeth. It does this by slowing down demineralization process of decay by actually making tooth enamel harder. In addition it aids in speeding up remineralization process.

(Source: Talwar , R & Talwar N (2006). Your Guide to Teeth Care. Sterling Publishers Pvt. Ltd.)

During eating, some acid produced by the bacteria in the saliva can causes demineralization of the phosphorous and calcium components of the enamel of teeth. Fluoride palys a big part in slowing down this demineralization process. 

For toddlers, depending on the kind of fluoride used by the dentists, they normally need to have fluoride applications every 6 months.  It is recommended that you check with your dentist to know and understand the importance of fluoride and to know the adequate amount of fluoride to take to prevent cavity.