If you are trying to make your son or daughter the Administrator of their own Club Nintendo account, unfortunately there is an age restriction and the person has to be 18 years old before that can happen, sorry.
So, You cannot post on the forums and such until you are 18?
Nope, a person 13 years of age and older can join these forums, so try having your son or daughter use the forum sign-in page at the above url with their Club Nintendo account username/password combination and it should allow the person to post here after agreeing to the "Terms of Service" pop-up that may appear.:)
But if no ToS message and/or still not allowed to post here, then you and/or your son or daughter may have to give a call to Nintendo's 1-800-255-3700 (6 am ~ 7 pm Pacific, everyday) Customer Service number, or use their webform to contact them, and a service representative will make it so that he or she can join and post here.