Netting not needed at this age. We took ours starting 3.5 and 7 years. Cat with comfortable lifejacket... No tether as if fall overboard, they could drown before you get them out/ release tether. And they will not be on deck if any significant weather.
Enjoy your trip!
If you use a tether, it should be secured so they can't reach the water. We do this with our son on our 43' sailboat. We tether him within the cockpit. He can't even reach the life lines.
I think at the ages of these kids though, a tether isn't needed. A life jacket and setting rules on where they can move, how to move and when would be enough.
