In order to be a collaborator of any of the Kid Blogger Network boards, you need to be an approved member of the Kid Blogger Network on Facebook. If you are not already, you may read over the requirements and see how to join here.
UPDATE (August 1, 2016): The KBN Pinterest Boards are no longer taking contributors. If you are a member of the KBN and know of a member on one of the boards that you’d like to be on, you may ask them to add you. Please do not ask KBN admins to add you.