In reply to Skinny:
I'd broadly say, if you're crossing the channel by day, then take the ferry as the kids can run around and enjoy that part of the journey. Last year we timed it so dinner and wind-down was on the ferry, which meant they went to sleep when we got back in the car for the 3-4 hour drive to Font.
The drive the other side isn't particularly interesting - we've done quite a bit of Googling and there's not really anywhere conveniently half-way along the journey to stop for half-a-day, especially for littlun's.
It's not a rule as such, but certainly guidance for <1 year olds is no more than 2-hours in a car seat at a time if possible.
We have taken the tunnel previously but this was a full evening/night journey - left home (Surrey) just after rush-hour, on to the tunnel, then on to Font. Long, boring drive, but the kids slept the entire way.
Last year, we did the evening crossing, then stayed overnight just outside Disneyland (lots of hotels in Marne La Vallee). You're only a 45-minute drive from Font then, but there's a fairly decent zoo nearby which we spent the day at, then on to Font in the afternoon for check-in.