High five to your wife!!...wish I had thought of the pool-inna-boat idea when my girls were that size (graduating college now)...great idea!!
For your 1st boat, you picked a great start w/a 21...as you're finding out, very versatile boat...w/LOTS of floor space...for a rear bench seat, you might consider one of these...
http://www.overtons.com/modperl/prod...t&merchID=4005
This particular one might not do as it looks like a permanent mount...but you get the idea...additional cooler/fishbox space, seating and should be able to mount so it's easily removed...there are others around like this, only not so expensive.
Destroyer's comments...following the law is one thing...being vigilant around your kids is more important...kids around water is a dicey situation at best and you're hearing this from a guy who RAISED two taking them boating/fishing every chance we could...and along the way, there were some close calls I must admit...eyes off for seconds can make the difference...my two are grown up and WATER-PUPPIES to the core!!...Ya'll have FUN!!...
