We love using a pack and play (play pen) in the salon for the kids. It is great as a sea berth for our smallest during nap time and even better as a kid prison for those short times when we can't be distracted, like when we're docking.
Also, safety netting on the life lines. Invariably, kids will want to look over the side of the boat. The net might stop them from going over...or the toys they are playing with.
Someone here once recommended a bucket of water and a few bath toys in the cockpit and I heartily agree. My kids love playing like that on a hot summer day.
Also, it's worthwhile checking to see what the minimum requirements are for young children in PFDs. The type and styles vary by state somewhat. I believe last year or the year before, Maryland made it law that children had to have the style PFD that had the neck ring until the age of 4.
And one more thing, a portable, battery powered DVD player. My kids love setting up a fort in the V berth and watching a kid movie, which in turn gives mom and dad some welcome down time.