We found the deuter kid comfort III to be the most comfortable pack for the kid, by far. The reason - it is effectively a chair (with a backrest) for the kid to sit in, with a footrest. It's the only pack I know of which doesn't compromise circulation in the legs. Kid faces forward and up high with a view. Sun cover, rain cover, mosquito net. Can't emphasize enough our kid never complained about long hikes. Only 1100ci of storage plus a couple of water bottle/slot pockets though. But it's built on an actual backpacking frame, adjustable for torso, good belt to carry the weight, good shoulder strap lift, etc. I like the idea of strapping a pod or dry bag underneath and it would work with this pack. But like any other pack, you wouldn't want to add weight out away from the body.
Now the kid is a little older, the new version of this is a Kifaru duplex frame with a Nomad. Kid sits up high between the wings, wings stay open but securely strapped. There is room for a small bag (ie. e&e, etc.) against the frame if you need. Depending on the type of trip and how much gear needs to be carried the "seat" is either a small box cooler, or a drybag/pod stuffed full, and secured with compression straps and/or a grab-it. I also have a cargo chair which can be mounted either below or above the gear which works well if you need a platform. I figure you can also accomplish the same setup with a cargo panel plus pouches but haven't tried it yet. Either version lets you carry as much gear as you can carry.