How rare is it for a normal, healthy child to be actually harmed (not just exposed) by lead in the u.S.?

Not so rare. Old houses are often painted on the oufside with lead paint, particularly in the inner city. Kids find that the flakes of peeling paint taste sweet and they get their lead poisoning that way. This causes changes in brain development that is not easy to spot and may easily be ascribed to other causes.
Lead. Lead has also been found in children's toys. So it is important to have lead screening done.