I would say Tunnel too. It's so much faster - and you don't have to turn up as far in advance as you do to check on for a ferry. Plus if you are slightly early, and there is space, they will let you on the next train.
Automated toll tags are great - look on APRR website. This is the one I have used for the past 3 summers in France. It is one with the lower prices for usage and deposit.
Your regular breakdown cover will probably not cover Europe. ( i was pleasantly surprised to find that DH's new BMW does though! ? ) . I buy a policy from RAC specifially for European travel - a comprehensive policy for our car and famiy size has cost umm.. I think £72 for 15 nights. Worth every penny if something does happen.
Insurance - mine does actually have full comprehensive cover for Europe. I have checked and double checked with the insurance company.
I would perhaps factor in two overnight stops. The roads may be clearer,and faster, but it is awfully tiring driving so far. Really not worth exhausting yourself. Look at Ibis Budget and B & B chains. Generally quite good, and just slightly out of the main towns, so parking is easy and free.
Pack a cool bag with water/juices (pre frozen, to act as iceblocks!) and sandwiches, crisps, cereal bars, fruit etc. And a pack of wipes in there for clearing up afterwards.
Agree with above to check out AA, with tips for driving abroad - the items you MUST have in the car - and rules of the road.
Plan carefully, and you will all have a blast.
Enjoy!