The size of your daughter's lesion doesn't have special significance to me. It sounds like this area hasn't necessarily gotten bigger, hasn't changed in color, doesn't have irregular borders, nor has it bled on its own. I am assuming it's just growing with your daughter as she does and that she is developmentally normal.
When there are giant congenital nevi larger than 20com, patients should be seen by a pediatric dermatologist for exact management. These are ones where the incidence of malignant melanoma is higher and an association with neurologic problems has been documented.
Controversy does exist concerning the appropriate approach for medium-sized or smaller congenital nevi, such as you're describing. Whether there is a definitive increased incidence of melanoma arising from these smaller lesions is not clear. In general, these are usually not a problem. However, there is some thinking that they may pose an increased risk over a lifetime for development of malignant melanoma, so the prevailing, but not universal, opinion is that consideration be given to removing these nevi prior to adolescence. Benign appearing flat lesions of uniform color may be observed regularly by your physician until later childhood, when they can be removed under local anesthesia. If there is any question, a dermatologist (skin specialist) can examine your daughter's nevus and give you the most current wisdom.