I sure hope they never make it mandatory to wear life jackets at all times. They are uncomfortable, so people just need to be responsible. I will put it on when travelling in rough water, but would not be a happy camper if I had to wear one in 30C weather with no wind. I surely don't need anymore liberal legislation to look after me.
I think your kid should go to the JP and plead ignorance and hopefully get a reduction in the fine and then learn from the experience.