August will be a good time to go. You will still be able to get space. However; you might not be able to get a balcony. I would recommend comparing the cruise lines air program (which would include transfers) to getting the air on your own. All the cruiselines have some type of children programs. Holland America and Princess have been in Alaska the longest. You might want to look for a cruise oriented travel agent. You can look here
and type in your zipcode to find one in your area.