In a study of over 1,900 preschoolers, drinking sugar-sweetened drinks on a regular basis (4-6 times in a week) more than doubled the risk of being overweight. Sugar-sweetened drinks include non-diet soft drinks and sweetened fruit drinks. The study did not look at consumption of 100% juice.
Colas and other sweetened beverages have been under fire lately due to the rising obesity epidemic. Some say that it is just the empty, extra calories; others say it is the high-fructose corn syrup. Some say that they are not associated with weight gain (a lot of research suggests otherwise).
Whatever the case, preschoolers, and older children for that matter, should not drink colas or sweetened juice drinks on a regular basis. 100% juice is a better option, but only 4 oz a day in younger children and up to 8 oz per day in adolescents. Low-fat milk and water are far better choices for children.