Did you take a look at the menus? If not, take a peak and see what the offerings are. You can find them here:
http://www.wdwinfo.com/wdwinfo/dining.htm
Regarding going back to AK after your 6:20 ressie at Whispering Canyon, you should check the hours for AK, it's not usually open very late.
I can't offer an opinion on Tusker House, we haven't had the chance to eat there since they changed it to a buffet (but we're going there for breakfast in May!). If you're worried about sticking around AK waiting for your ressie, see if you can get a late lunch. If you plan on staying for the parade, it's not until 4:00, so you could always try to get a ressie for just after the parade.
Whispering Canyon can be a fun place to dine. If you get a really great server, you will have a lively dining experience. We like the food, but don't go there for the food. Does that make sense? So, if you think your family would be up for the "fun" of Whispering Canyon and the menu looks ok for everyone, then go for it!
I was probably no help at all! But, I'm sure whatever you decide is right for your family will be just fine!