You shouldn't necessarily just wait it out.
Phone your pediatrician if your child has signs of dehydration, including a lack of tears when he cries, a dry mouth, no wet diapers in six hours for babies, or no urination for 12 hours in older kids. Because babies are particularly vulnerable to dehydration, stay in touch with your doctor if your child is 6 months or younger. Also call the doctor immediately if your child has projectile vomiting, bloody or greenish vomit, diarrhea that's bloody or has a jellylike consistency, a tight distended abdomen, shaking chills, or worsening pain. These symptoms can signal anything from a tear in the esophagus to a bowel blockage.