This depends on the age of the child. Younger children who are dehydrated can deteriorate rapidly, so it is imperative that they be seen as soon as possible.  It is also important to know the nature of the vomiting: is it projectile, is the vomitus green (which means it's coming from a place further than the stomach and this could mean something more serious), or red (may be blood or it might be just the color of the food), or black (which might be digested blood, so it might mean something more serious), and how often it occurs. Older kids can tolerate vomiting issues better, but it is still important to talk to your doctor, and you might be asked to wait and see, or see your doctor immediately.