I started this post.  We just got back from our trip, which was just plain excellent.  We went with a Souris River 18.5 three-seater.  Me in the stern, wife in the bow.  2 packs in the open compartment behind the bow - one of which was a softer pack with clothes, sleeping bags, etc.  6-yr old sat on top of the soft pack.  8 and 10 year old shared the 3rd seat.  The 8 year old sat backwards on the seat, 10 year old sat forward and paddled.  That allowed me to put another good side pack just in front of the 3rd seat but only taking up half of the space.  Total of 5 people, 3 big packs and 2 small backpacks for the trip, plus  fishing rods.  Tight fit but we made it comfortably and were able to move at around 3 mph on the water.