Yes, everyone in your party must be present to get a tender ticket, and get lined up. I have seen people with smaller children, just grab fruit and such, bring it to the line and let them eat it while they are waiting in line.
No, they do not use lifeboats. There are companies in Grand Cayman that are hired to bring you back and forth. The boats are pretty good size, not to big, not to small. Smooth ride. You can get some great photos of the ship as you are coming and going.
The tender boats are available to go back and forth all day. On the dock side, there is a canopy, and I believe there was water to drink, if you have to wait a few minutes for the boat going back. All is very easy..so don't worry.