I think you'd do well with a Souris River Quetico 18.5' -- I'd envision the kids sitting in the center on one seat (you'd have to rig that up somehow) or possibly the 9yr old in a seat & the 5 yr old sitting on packs.  I've considered a MN IV but never used one -- I suspect it is a lot faster than a SR Q 18.5, but also harder to turn and a bit less stable.

I've always rented and have used SR Q18.5's twice & they're pretty light but very stable (even for fishing) & can haul a lot of gear.

For comparison, I've used SR Q17's also & they're noticably faster than Q18.5's -- The Q17 probably much more like your MNII in how it cuts through the water.

I'm sure others with more expreience than I will chime in also.

Best of luck.  -Tom

Of the possibilities listed, I'm only familiar with the Souris River Quetico 18.5' and 17'.  A Quetico 18.5' has lots of room, and should suffice for your journeying.  However, if you love the MNII, you might prefer the MNIV.  The loading should be done depending on wind conditions to get the stern or bow slightly heavier (lower in the water) towards the direction the wind is blowing from.  I like to bring a large day pack that weighs about 30 punds, that can be stowed behind the stern paddler or moved to the bow when paddling in a headwind.

You may want to also consider a Bell Seliga...I have run trips out of it with similar weights involved and was happy with it's performance, and the stability is terrific, easy on the portages too