Some of my grandchildren have gone sailing with me (on a Hobie Wave) before they could even walk. But, of course, they will not remember it.

Your 5-year-old WILL remember it, so make sure it is a pleasant sail -- no hull-flying. Make sure it is a day where you can put the 5-year-old on the tiller without worrying about a wrong move (but don't let go of the mainsheet).