A few suggestions:-
Give them a job to do – this requires sensitivity, as some confident kids will love to take over helming straight away, but others would be put off for life by the idea. “Keeping a lookout” can be adapted to circumstances, and you can easily concoct a situation in which they save the day, e.g. by you steering a bit close to a mooring buoy “by mistake”.
As young teenagers our two enjoyed a commando-style raid on Padstow a few years ago, launching from just over the estuary at Rock. I can’t say too much about this for legal reasons 😉 .
Incorporate food/drink at some stage. A picnic or even a pub meal makes for a memorable day out.
As someone pointed out on this forum a while ago, very young children (and boat owners) are put off by flapping sails, so avoid this if possible. Toddlers might be happier just playing on the boat at the beach for a few minutes with no sails up and no intention of going anywhere. My little nephew didn’t even get into the boat last year – maybe next time!