I don't mean to stomp on your fun, but it's my guess that almost no one here would use a pack that has attachments for other smaller kids packs. I backpack with my kids, but they have their own lightweight single compartment packs. If I ever needed to carry their pack(s) I would simply lash it on to mine or wear it backwards on my chest. I wouldn't carry an extra once for the unlikely convenience of carrying another pack. As far as compartments - a single compartment pack with a few lightweight stuffsacks is really no inconvenience at all.

I would recommend that you find a more mainstream backpacking site. Most hikers here will likely not be able to relate to the problem that you have presented - sorry.