My son started sailing with us when he was 4 months old. We never used netting. He had to wear a harness and tether whenever he was on deck until he was a strong enough swimmer to be able to take care of himself if he fell overboard. At that time, he didn't have to wear the harness, but he still had to wear a pfd whenever he was on deck or on the docks. You'll know instinctively when they are mature enough so that you can let them walk around the docks without a pfd, somewhere around 11-12 years. IMHO, the netting is relatively useless. A toddler is too unsteady to be allowed on the foredeck unattended. Once they are able to walk, they quickly become able to pull themselves over the lifelines. You need to keep him or her close to you until the child can swim, and the only way to keep them close is with a tether, so they can't get away in those moments when your attention is drawn away.
It might sound like you'll miss out on a lot of fun by having to keep them so close, but my son was frequently in the cockpit when we were racing. At about 2 years, he plopped over the side just as we were rounding up to start a race. I reached down and grabbed his tether, and swung him back into the cockpit, and we hit the start perfectly. I often think that if he hadn't created a little drag and slowed us down a bit, we might have been over early. You'll still have plenty of fun. It'll just be different.