You may know someone in Hawaii or maybe a Hawaii contact will show up but if not, you could call 1-800-GoHawaii and ask them to help, if they won't you could ask for a visitors guide from them which the kids would receive from Hawaii and also enjoy seeing the pictures from the magazine.
www.gohawaii.com/about_hawaii/visitor_guide
For Alaska, the same thing you could either call them or ask for some brochures at there website
www.alaska.com/brochures/search
or
www.alaskachamber.com/artman/publish/staff.html
Another thing, if you want to ensure postcards, you could email each states governing office or most have 800#'s, I am sure they would love to help. Just search each states government on a search engine and Wa-la!
Another one I thought of you could Fax the President! They need 14 days notice to send you a greeting.
FAX: 202-395-1232
MAIL: The White House
Attn: Greetings Office
Washington, D.C. 20502-0039
I have a friend who is in a Letterboxing club, he passed on the info to his club, this is right up their alley.
Let us know how it all works out!