Teach them life, we have a day or two doing what they want, and a few days doing what we want.
Life is a bitch and the sooner the little feckers learn, gently, the sooner he better.
Don't give in, or they will expect it for ever.
When it comes down to it, they have to do what you say. Wear your life jacket. Go to bed, it's grownup time. DO AS YOU ARE TOLD.
Being a grandad and being soft undermines the parents. Being a parent is about training your kids to be a parent so you have to followup by being a parent to your grandkids.
I grumbled at my Dad for spoiling the kids once & he looked me in the eye; "I didn't spoil you did I?" No, dead right he didn't! "You don't spoil 'em do you?" No, We don't.
"Well some beggar has to" he said.
I had no answer to that, and I think he was right. So go ahead, enjoy spoiling them, they won't damage if the parents don't do it too. There is no reason why you shouldn't run the trip entirely for their benefit. They learn quickly so show 'em a chart, where you are & where you are going. The Chart Plotter will be a great toy if they can spot the bouys & rocks as you pass them, my lad's eyes have always been really sharp so he was always chief bouy spotter.
Sitting on the bow (in a harness with legs either side of a stanchion) counting jelly fish was fun with 2 of them competing. Keep 'em below at times of high activity - reefing/ sail changes, picking up or casting off moorings etc. But let 'em out again as soon as you can.