Hi Sunseeker.
You don't all have to go off the ship together, BUT, if your kids are in the Kids Club, then there must be one adult left on board. On the signing in sheet you write down whereabouts you will be in case they need you.
The policy is that you should not leave any children on board unsupervised.
When we went in the summer there were people leaving young teens on their own whilst they went ashore. After seeing several accidents, people slipping and banging their heads, and fainting in the heat. I am not sure I would consider leaving older kids on board on their own.