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