Sounds like a a lot of the same requirements I had when I got my first (and only) kayak. I ended up going with the Ocean Kayak Frenzy Sit On Top. It is very stable, I can stand up in it, but I wouldn't fish standing up. Its only detriment is that its a 9 ft yak, which makes it a little slow going, but its by no means hard to paddle. Its a great fishing platform, and its also a great surf playing machine. I got mine from a Ski Market Warehouse clearance sale a few years ago for around $300 dollars, down $100 from the regular $400. So its pretty inexpensive.
It is a sit on top, so that means I wear my waders 75% of the time in it, but that doesn't bother me at all. And since its a sit on top, there is no worry at all of flipping it, filling it, bailing it, or sinking it. Its plastic so I can drag it, kick it, abuse it and the like. I actually just picked up a new carbon fiber paddle from an auction here donated by www.capecodekayak.com
and it has made a huge difference in paddling.
Most of my paddles in the boat have been the 2-3 mile range, but I have done approx. 7 mile paddles with the Roopster and Penguin. As far as putting a youngster in there, that would be tough. I once tried to paddle out to an island with a roomate on board, but that's another story
. A smaller child may be doable.
Let me know if you have any more questions, and if you're near Boston at all, maybe we can get together so you can try it out. It is better to test them out before you buy!
BTW---I'm 6'3" 210lbs and its been no problem for me.
just read that your in Manchester...I work in Peabody and paddle there after work quite often...