We started our first son sailing at 5 weeks old if I recall. That first season of sailing was great. He was basically always in a car seat, booster seat, lap or bed. We had a safe spot in the cockpit for the car seat when the weather was calm and I made some straps up to lash down the car seat when underway and he always slept when the engine was on. So much so that once, at anchor when he was fussy, I turned it on just to get him to sleep. (sorry anchorage mates, but it was probably quieter than the screaming).
The only bit we were nervous about was getting in and out of the dink. For this we opted for the kiddo to be strapped tightly in a baby carrier to his mom and the dinghy was tied alongside bow and stern with me steading it. Being strapped to mom meant not wearing a lifejacket for the kiddo, so we would have been at the mercy of the USCG if stopped. We felt it was the safest and best thing to do though.
Kiddo slept with mom in an arrangement where he couldn't roll out of bed and we didn't push the season so it was warm enough at night.
It was the second year when things really got difficult. Once the little one CAN move about, that's all he wants to do every waking minute. His balance wasn't the best and he was always falling over and banging into things on the boat. We then had another crew arrive and last year's season, with a 2.5 year old and a 1.5 year old was our most difficult so far. We even aborted our favorite trip (4th of july) half way through because it was going so poorly.
Here's some of what we learned sailing with toddlers:
1. One MUST be able to completely single hand the boat. Taking care of 2 kids down below is a big job and the other parent must be self sufficient up top. This is where docking practice and confidence help as well as an autopilot.
2. The kids need to move so time underway must be short. Short hops are best and timing with nap-time sometimes worked. I've heard that getting underway first thing in the AM before they're awake also works.
3. Be extra mindful of their comfort. We let our oldest wear his lifejacket all day at his request. We loved that he liked it so much. He ended up overheated from the sun and everybody was miserable afterwords. We try and keep them from getting too hot, tired, hungry etc.
4. Docks. We used to anchor out all the time. Now, we use docks much more often. Being able to instantly step off the boat and run off energy is key. Even when your dinghy is easy to use, it's a production to get everyone in it, and out of it on a beach. With docks, it's instant and shore time can be as often as needed.
5. Portable TV. Purists we are not any longer. Parental Survivalists we have become. I never thought I'd see the day, but I'm shopping for a wall mounted TV now for my boat.
6. Load the boat with adults. 2 toddlers VS 2 adults = the adults are outnumbered. Bring reinforcements.
Make sure to take a couple shorter trips to gain confidence and see what works for you. What we found though, is that kids change so fast that even once we thought we had our "boating with kids system" all dialed in, the kids changed so much that we had to go back to the drawing board. Be flexible and have options whenever possible and expect cruising to be differently challenging and differently rewarding.
Having said all that, go out as much as possible before they start crawling! It's when things are the easiest! :-)