Gonna have to recommend 16' raft.... Great thing is....in about 3 years or so, you'll be adding a ducky to the fleet because your oldest will want the independence and a wilder ride (I would suggest a double Duck...your kids will LOVE IT)....by then your wife will be proficient on the oars and your two youngest will know their way around the raft pretty well....
Then you buy a 12.5' cat for yourself by selling the idea to your wife that it's "easier to row" (which it is actually) and it'll help progress HER skills. You're also planning longer multi-day trips for the whole fam by then so a second boat is needed for all the extra gear you've accumulated (Drybox, coolers, groovers, etc..) anyway.
Your second oldest is thinkin'....."kayaking sure looks like a lot of fun"....and you're going to be looking for pool sessions over the winter so he/she can work on the roll....then you'll have a kayak in the fleet.
But wait!!! He/she can't learn to kayak by themselves....so you and/or your wife think it's time to give it a try too.....another kayak in the garage.
OH MAN....it never ends!!!!!!!!!!!!! Just casual observation and predictions....I've seen it happen.
Best activity EVER...I'm pretty excited to be raising my boy in the river community.