If you need any advice on things to do let me know, I frequently go to that area.
We usually go on one of the Brittany crossings, then it's around 8 hours actual driving from those ports. I've done it in a variety of ways. Arrived in France at lunchtime and driven through until reaching there at nearly midnight. My kids get bored on ferries also, on the whole unless we're sleeping then the fast 4 hour one is about as much as they'll take.
I've slept on the St Malo crossing on the way out then gone back on a Cain or Cherbourg crossing evening or night crossing. This worked really well last time. We fed the kids (2 and 6)before getting on the ferry, then went to the bar. They had a dance and watched a magician, then went to bed late but too exhausted to cause any probs.
We've also taken an evening crossing, got off and driven an hour or so to a Formule 1 hotel. www.hotelformule1.com/formule1/gb/cartographie/france_no.shtml Pretty sure we've been to Avranches and St Lo after getting off at Cherbourg.
You wouldn't want to spend too long in one but they're cheap and acceptable to just sleep. The rooms are a double with single over the top. However you let yourself in when the reception is closed, noone to notice the extra child and a Ready Bed .
We've pretty much taken our own food for most of the trip, if you plan right it's possible. Last few times I've taken our little gas picnic stove and cooked some sausages and that packet flavoured stir fry rice! Then boiled up water for coffee to take in thermos cups. Had quite a few nice comments from hungry Dutch and French families.
There are a few really big interesting service stations along the way for a long break. There's one with an agricultural museum, doesn't sound exciting but it's interesting buildings and areas to walk for youngsters. I'd have to ask my parents.
I've not yet done the Bilbao crossing, tempted but worried about rough sea and sickness.