I practice in a multi specialty clinic as a pediatric primary care provider, where I work side by side with a pediatrician and family physicians.  In our pediatric department, we see children birth through 18 years.  When they turn 18, we feel that since they are becoming young adults, their health care needs change, and those needs might be better addressed by a family physician.  Therefore, we refer them to a family physician at that time.  Also, when our adolescents become pregnant, they are automatically referred to a family physician since pregnancy is not a pediatric issue.

Family physicians are trained to take care of children and adults.  However, some may decide to see children when they turn a certain age.  For example, some may not be comfortable providing health care to infants, so talk with your family physician to find out his or her comfort level in treating children.  On the other hand, you have to decide whether you feel more comfortable taking your child to see a pediatric provider (pediatrics is a sub-specialty), or a family physician.  

Best regards.