I would not put a child in one. Inflatable life jackets do not work 100% of the time. Children are precious and not worth taking that risk. Regular lifejackets always float. I know a lot of kids squirm and hate them but they have to learn that sorry, that is how it is until you get bigger.
I agree with David.
However, it depends on the age and weight. If he/she is above 90lbs then there are automatic and manual inflating PFD's with CO2 available, with the pull of a string...poof. But realistically just keep them in the vests... its safer.