I'd go for Gandspette, personally. It's our site of choice for an early ferry - handy for the autoroute, good sanitary facilities, especially for children, a pool if it's still warm enough, a restaurant which does good value good - and some cracking regional specialities, and it's geared up for people setting off early.
You need to be within 20 minutes or so of the ferry, otherwise you'll have to be up at the crack of dawn, and Gandspette meets this criteria.