Here at L.W.A.H.Q. we do some pen pal matching for kids, but the majority of our members are adults. If you're looking for a pen pal for a child, we recommend checking out the Secret Society of Letter Writers run by stationery company Mr. Boddington's.
"Master Boddington welcomes boys and girls from Brooklyn to Tokyo to Moscow to Buenos Aires to join a Secret Society to honor and protect the hand-written letter. Members will receive periodic post from the headquarters regarding important news, secret messages, letter writing projects, pen pals and the club’s annual Who’s Who. The club is absolutely free and solely exists for the amusement of children who love to write. "
The sign up is online and here's the fine print from them. Are you or your child a member? How do you like it?