One of the best things is to simply take them to an outfitter or two and see what fits them well.
When my oldest started hiking with me at age 7, the thing we found at a local outfitter that worked was the Granite Gear Vapor Day Ki - a 32 liter woman's day pack with hip belt.
While this particular pack is no longer manufactured, I use it to point out that you might find something other than a "kids pack" that will work.
And just because it's listed as a 'kids' pack, make sure you still pay attention to the weight of the empty pack. There are some kids packs out there that weight 4 lbs empty. What I've read is that you should limit a pack weight to no more than 20% of a person's body weight. That means if you are trying to hike with a 50lbs child, a 4lbs pack already weights 40% of your total limit.
One pack I would suggest you try to look at (perhaps for the 8yo) is the
REI Flash 45. It weights only 2lbs with a 45L capacity for the medium size.
Again, not something to specifically recommend to you, my most recent purchase was the REI Flash 52. This is a woman's pack that my 12yo is currently using. The point being... don't limit yourself to 'kids' packs.