G'day, not sure if you are aware of the regulations regarding PFD's in boats, but a "swim vest" such as the Wahoo one linked here is not only illegal but dangerous also.
A PFD for boats must be classed between 1 and 3. 3 being for enclosed waters such as a river or lake for instance and 1 being for open water such as the ocean.
A swim vest on the other hand is designed to give a child aid in buoyancy for easier swimming. It is certainly not a life preserver by any means.
The link D4D posted is great if your child is already a capable swimmer or understands water to a degree but will not aid in assistance swimming due to automatically inflating once contact with water occurs.
I have a young child and another on the way. I am also a boater (ski boats, tinnies and cruisers) and have a multitude of PFDs for my child for different uses. I also have a separate swim vest for swimming in the pool or close to shore on the lake.
Whilst I understand this may be impractical for most people, its only a couple of hundred for the appropriate safety gear for your child/children.
The reason behind a separate swim vest is that an actual PFD is rather bulky for "fun" swimming....
For a young child on a boat
http://www.chsmith.com.au/Products/Oceantot-Infant-Baby-Life-Jacket-Level-100.htmlfor a larger child on a boat
http://www.chsmith.com.au/Products/Triton-Child-Life-Jacket-Level-100.htmlSwim vest that we use
http://www.swimways.com/swim-vest-p-278.aspxWhilst I understand some say that it is better to not use swim aids so that your child does not become dependent on them, I think that for piece of mind its a no brainer.
Cheers
Evo