Depends on the child, really. We started sailing with my older daughter when she was just about 15 months or so. Lifejacket was on, but otherwise we didn't do much else. She's a good sport in that she falls asleep on board pretty quickly (partially because we give her quarter of childrens Dramamine to avoid occasional seasickness, you never know with kids). So, we just tuck her into the corner of the cockpit with lifejacket under her head and for the next 6 hours or so she's catching some zzzs. Make sure child lifejacket has support behind his head - it is both a safety feature and really convenient as a pillow
The little one is 8 months now and pretty active, so we try to stick her in a v-berth where she is free to roll around
In the cockpit unless coaming is really high, active child might be a problem - and it isn't so much about harnesses. Harness will not keep a toddler from falling on the floor or bumping his head.
One thing that worked for us (for the older one but we plan to try it on the little one too) is a kind of a "seatbelt" we fashioned out of nylon webbing and snap buckle, and an adjustable foam seat (the one where the top inclines). Keeps her relatively safe in one location, if needed.
YMMV but above all don't worry too much - kids seem to love boats and do better then their parents occasionally