For the trip, you have one night booked - great. I would be inclined to do two nights, but your place is quite short of the coast so you will probably be fine with one night.
Take the Autoroutes - they have tolls, but the tolls are not too outrageous, and you really do get what you pay for.
There are lots of rest areas, called Aires. Some are literally a toilet and picnic area, and others are full-blown services. The signs on the side of the roads tell you in advance what you are going to get. Some of the picnic area types have a play area, and these are great for non-stressful stops. The bigger places tend to be heaving.
You really don't need a lot of cash en route, as everywhere takes cards, but if you want a coffee, you will need coins for the machines.
If you are Satnavving, beware that you may be directed onto very minor roads and even farm tracks. It's a really good idea to know where you are going generally beforehand.
If you have a high car (ie MPV with roofbox, over 1.8m), you will be restricted in lots of parking places.
If your car takes diesel, you need to look for Gasoil or Gazole. It should not be a problem at all - same as UK.
Police are very present on the Autoroutes, so make sure you stay within the speed limits - 130km/h for dry weather, and 110 km/h for inclement. It would also be a good idea to check the speed limits on the various minor roads - I never quite understood the signs as they used the white circle with black diagonal line all over the place, but there is a definite difference when you are on a Route National, and in a town.
If you eat in a regular restaurant (not a motorway place or fast food), expect a lunch to take 2 hours. You can get cheaper wine by asking for a pichet. It is fine to ask for a pichet de l'eau, and you can get sirop (squash) for your child instead of fizzy drinks. Many restaurants will have a menu enfant which is fantastic value.