The queues can be stinkers - sorry to say usually Saturdays in peak periods are bad to get into the resorts, and then on the day/evening that people are leaving on the way out.
But not much you can do about that other than resign yourself to it and make sure your car is well stocked with all you need (food/drinks/entertainment). In the past, we have pulled the dc out of school on Friday and stayed near Moutiers that night so we could get in the resort early on Saturday before the crowds.
If you can't do that, definitely go on Friday night and travel as far as you can - the M25 if you need to use it will be your biggest issue that evening.
Google Maps/Via Michelin are quite reliable in terms of driving time, but remember that doesn't account for stops.
How good are your dc in the car? Do you have a DVD player that will keep them occupied?
Can they/the non-driver read without feeling sick? If so, take books/magazines in the car too as well as blankets and pillows - our dc slept most of the way through France, in both directions on daytime drives.
Be careful with SatNav - one route (as we found out to our cost when we ended up on the Peripherique in a Monday lunchtime jam) will take you on the outskirts of Paris - so check what route it's planning to take you and go east of Paris, not directly around it!
LovelyJudy - thanks for the tip about the motorway toll card, didn't know about that, but it would've saved us at least half an hour on our last trip.
OP - have great fun! We much prefer driving compared to the cost/hassle of airports and airport transfers and it's lovely knowing the holiday has begun as soon as you start the engine to leave home, and you can stock up on wine French produce to take home.