I've used both the Kelty pathfinder and explorer (older model), the explorer is more comfortable for heavier loads/longer time but both are very good. Avoid the models with "climbing rope" handles as they are coarse and uncomfortable on the back of your neck.
Also, be careful my son is 4 1/2 and got so used to the pack that he would rather be carried than walk on a trail. It was hard to break him of the expectation to be carried all the time.