Scott, we stayed recently at a hotel in Campo Santa Maria Formosa and had a morning flight out of VCE.
We just told our hotel the flight time and they arranged a water taxi to pick us up at the proper time, very easy and relaxing way to end or time in Venice.
For a party of 3 (which we were as well) I didn't think the water taxi was overpriced for the service it provides and is truly a luxury ride.
You do have a bit of a walk where the taxi drops you off to reach the airport terminal, but it is an obvious and flat walk on pavement.
The taxi can come to the edge of the campo you are staying in so a couple hundred yards from your hotel you can be picked up.
Making your family take a vapporetto or long walk to Piazzale Roma so early in the morning and then go to the airport on public transit from there all before waiting on lines and going on a long flight is one bad way to start the morning. Pay the extra expense and be comfortable, the family will thank you for it. May even get some good sunrise photos from the boat as you leave town.