Due to the fact that I am also researching a cruise (for 2008 though) I cannot gaurentee what I tell you but this is what I have learned.
1. Yes, Auguest is wonderful for young ones. In June it stays light very late into the night and may be hard to get adjusted to. You will see the fall colors in Alaska. Check weather.com to see the difference in climate. Alaksa cruise do book up quickly but you may still have time booking your cruise. Prices on flights is another story.
2. Royal Caribbean and Princess and Carnival are great with kids. All the major cruise lines have set up programs but Princess has set up special excursions for kids.
2. Although I do not no of any particular site. I would be glad to research the cruise for you but will be unable to book since I am not a travel agent. VTG is a great website too!