This is the one my son has: Toddler Pursuit CGA
He's 2 1/2 and about 30 lbs. We've had him in this one since his 1st birthday. The neoprene is way more comfortable for him than the first PDF we had for him which was nylon. We found it at Costco for $30.
As for when to get the kiddos on the river, we took our son on the Roaring Fork from Carbondale to Black Bridge for his first run at about 10 months (mid-summer) and then took him on a 3-day Ruby float for his first birthday. Last summer we took him on a Rio Chama trip and on the Pumphouse (late summer) and he did fine on both trips. This summer we're going on a Deso trip and probably a few day trips here and there.
I'd highly recommend putting your kids in this class if you haven't already:
Baby Swimming Lessons, Infant Swimming Instruction, Survival Swimming Instructors
We put our son in the class at 6 mo. and he learned how to float by 1 yr. Now he does what they call swim-float-swim. To 'graduate' from the class, you put your kid in jeans, a jacket, shoes, etc. and they basically throw them in the pool to see if they can rollover and float. It's amazing to see how the instructors can get an infant to actually rollover, calm down, breath and float. I can't recommend this strongly enough if you're going to have your kids on the water. I realize that a pool and whitewater are two completely different things but you have to start somewhere.