Immunized child. If your child has his immunizations up to date, he should not suffer any ill consequence as he is protected if the exposure is within the expected time of vaccine protection.
How old a child? Age is certainly most important. ..Tough babies are at biggest risk of serious disease and consequences. After age 5 or 6 yrs pertussis infection for the most part no more than a typical uri. And if fully vaccinated risk of catching disease is pretty low. I hope that helps.