Some airlines, and I don't know if Alaska is one, will not let a parent sit in the exit row if their child is on the plane, even if on a separate reservation. The thought being that in an emergency you will try to get to your child, no matter what instead of fulfilling your exit row duties. If that was the reason though, and again I don't know if that is Alaska's rule, it should've been explained to you more clearly and politely.