Hi, JoyAlison. You look fantastic! I can't believe you're a mum of five, you look in much better shape than me, with all of my zero kids...;)
I would probably agree that you're a rectangle, but we'll see what Angie says. There is basic info on rectangles here:
http://youlookfab.com/2006/05/.....rectangle/
For dresses, specifically:
http://youlookfab.com/2007/04/.....ectangles/
The only thing I'd say is, looking at the styles recommended for rectangles, some of them seem a bit "straight" for playing/running around with kids, if you know what I mean. I know it drives me crazy to wear a more restrictive skirt if I'm trying to be even remotely active. Choosing a slightly fuller or A-line skirt might be more practical for you.
As for specific dresses, I'm afraid I can't be much help, as I don't live in the US. Good luck, though!