i have sailed with children as young as 3 weeks old
its all very do-able and enjoyable.... we basically strapped the car seat in for if its rough.
we used a highchair thing that you strap to a chair but used it on the floor...
depends what size / style boat you have.... i reckon you need at least 40ft to be comfortable though!
also found motorbaoats much easier than yachts.... cause you get where you are going much quicker... ie a channel crossing in 3 hours rather than all day...
also pick your mariners carefully.... some a much more 'baby' friendly than others (ie many have large sinks that you can easily use as a baby bath and babychagnung areas in the showers etc.... where as some "dont" at all!
you can easily use a life jacket... we did on a 3 week old...
also my top hint is.... Put on Reains.... then life jacekt then the strap that you hold on to (after the life jacket) makes holding onto the child safely MUCH easier.... in the mariener i put a wrist strap onto the reins so that i could not accidently drop the reins and he was always suitably attached
LOADS of people have lots o happy times sailing with babies and young children (and dogs) it CAN be done and is done by loads of people!
be sure to research your mariners first!