Woodstripper, they make good PFD's for kids and/or you can modify them as well.  My wife and I did some of each.

Things to look for -
1 Make sure that they have a head flap or pad that helps keep their head out of the water ( this is mostly for infants ).      
2 Make sure that they will turn the kids face up if they go in the water.    3 Make sure that there is a grab strap near the head so that you can pluck them out of the water if needed.  
4 Make sure that there is a strap between the legs so that the PFD doesn't slip up over their head.
Some of our kids PFD's we added these things, some of them we got most of what we wanted right out of the box.

One of our rules for our kids until they were about 7 or 8, and could swim well on their own, was that when they were out of the tent they had their PFD on PERIOD. Then it is a lot less hassle trying to keep track of them and worrying if they are near the water or not. If you heard a splash, they were too close, but they learn fast that way. Wearing their PFD's full time also acted like body armour for the inevitable falls they took. We also allowed each of our kids to "fall out" of the canoe before they were 2 years old. You can only tell them so many times to quit leaning over the side so far. None of my kids fell out more than once.  

My wife was pregnant to some degree on a lot of our trips in those early years. 7 months was the farthest along she ever was. That was a 16 day trip with 20+ portages including the 950m out of Brent and the 1000m into Pond. We also swam the Brewer Rapids that year for fun.
I know - we are crazy - but man do we have so many great family memories from the Q, and all of our kids have turned out so far...no damage other than a craving for the wilderness each summer.
Gene M