We took the 3 hour whale watching tour with 4 adults and 4 kids (aged 8, 9, and 2 11-year olds) and it was a hit overall. The naturalist was very enthusiastic and that made me feel even more excited about what we were seeing than I already was feeling! The kids (well, all of us really) were kind of over it by the 3rd hour (we began making jokes - "Oh no, not another whale!") but I can see how that 3rd hour may be needed if the whale weren't cooperative for the first two. We saw 16 whales and it really was amazing.
Tip: Check for a Groupon! They seem to offer Groupon deals quite a bit - we used one and saved money.