I would suggest late seating as there won't be as many kids at dinner at that time. Crystal has a new program this year for the Alaska sailings where kids sail free, so expect lots of kids on board. If you've cruised Crystal in the past, the idea of kids on board will be something rather new. Generally speaking, Crystal usually is a line for adults, and not many kids, but with this new program, I have the feeling that kids will be taking over the Alaska cruises.
The White Rail Pass is a lovely trip. I also can highly recommend the whalewatching trip in Juneau. If you don't mind helicopters, any of those tours which go on the glaciers are really special. And if you like to fish, salmon fishing trips are fantastic---and if you catch one, they will bring it back to the ship and the chef will prepare it any way you want and serve it for dinner. One of my tablemates caught a salmon and it was the best I've ever tasted.
Yes, take a large table for 8 at dinner. That way you can have a really good start at meeting people. I always cruise Crystal as a solo (I'm doing the maiden voyage on the Serentiy in July) and I think Crystal does a really good job with singles. For a nice evening activity, try the Avenue Saloon. Lots going on there in the evenings.