Thats hard, I wish you the best in getting them interested...sharing our experiences may help...
For us, we wanted a boat thats comfortable for us and our two kids. Its safe and provides enough room for all of us to have our own space, something that I think is very important.
We were able to start them very young, Elizabeth was born in November, Olivia in February, both were sailing with us the following Springs, so you could say they have grown up on the boat. We provide many things they can do while we're sailing. When its hot we may fill a tub for them to play, they love puzzles and games, much the same things they play with at home. Although we do not have a TV on board, never will, I hate to admit it, but a DVD player is useful, especially comes in handy when the weather is not so good and they can go below and be entertained while we sail the boat. When we finally anchor somewhere we usually attempt some fishing, or go for a dingy ride and explore a nearby cove or creek...this is one of their favorite things to do. Beach combing is a family favorite. These are all things they look forward to doing when we go out. There are places we have been several times now and when we plan to go there again they remember the fun times they had. Getting there becomes part of the fun. For example, if we tell them we're going to go to St. Michaels they know we're going for ice cream later and they get to play on the playground "ship". If we go to the Inner Harbor they know we're going to get ice cream and they can go for a carousel ride. Do you see a theme here?
The oldest is starting to help sail the boat, she often wants to take the helm or help grind some winches, I feel getting them involved in sailing the boat is the future for us. This has been the hard part for me, but I am learning to chill and not worry about the sail trim so much
Weekends are pretty much all we can do right now, although we do plan a couple weeks in June or July, but someday I hope we can spend the entire summer on the boat. Time on the boat is the best family time of the week for us, distractions are wildlife, cooking, eating, reading, etc...not TV, phones, Internet, etc...
Going somewhere is key I think, just going out for a sail is not only a lot of work for little time, but not so interesting for them...its all about the adventure
We are very fortunate, and hope we can continue to do what we love.