Bigwig,
Will you consider an external frame pack? With all the gear you carry, it might work out better. If so, check out Cabela's Alaskan frames and bags. You can make your own sack to fit the bag if you can sew.

Are you open to suggestions on how to reduce weight for the young ones or do you pretty much have your system set? For example, the 5 & 2 yr old could probably be covered by a quilt rather than having to carry two separate bags.
But, like I said, if you've got your system set and only need something to carry it all then no need for going on.

Have you read the threads in Backpacking with Kids forum down below?

We'll be happy to help if you want.

Tango