Hi J., we have always used Jim Nelson photography. . .they did our wedding and now do all of our family portraits. They do great location shots. Some of our older shots are on their website, too, and you can see them on this page. . .http://www.jimnelsonphotography.com/old/index.htm. We're the family in jeans and white shirts. There are 3 shots.
They are also picky about the shots they will let you blow up to that size. We had some GREAT shots from one sitting and when they blew them up to the size you're talking about, the boys' heads were moving just enough that it showed. They reshot the pictures without cost to us and we chose all over again.
I HIGHLY recommend them!! Oh, I would also time it so that you're either early enough before your 16 month old's nap or after so that the kids are fresh and in good moods.
Good luck!