Well I have 4 kids, 2 are grown and starting their own things. 2 are at home yet and have the on again off again love affair with fishing/ hunting.
Early season in the boat, they are both all in, heat of summer they are both out, later in the fall back in.
The key is to keep the boat full of snacks, possibly a game or music or something, and if they say they want to quit, call it a day.
If you over do it, they wont want to do it.
My youngest (only son) will put more into it, and he loves to come with me and the dog hunting, deer hunting, ice fishing sometimes.
The golden rule is if you know its going to be a long and tiresome trip, or event don't let them come with. Worst thing you can do is make what should be a great day a terrible experience. Also if they goof up a netting job or hook set, never be negative about that either.
Only positive comments and nice suggestions should ever be made while out having fun. I watched a friend tare into his son on an outing one time, and he now cant understand why his boy doesn't ever go fishing with him anymore.