Lesley appears to have answered your question. I reckon St Malo is approx 175 miles from La Garangeoire. So allowing 4.5 hours is giving yourself plenty of time. I have never driven around Rennes or Nantes in August, but if you read the signs correctly and get in the appropriate lane early enough you should have no problems. You do not go through Rennes or Nantes but around the ring roads. Rest areas are very frequent and far superior to anything we have in this country. They cater for caravans as well as children. If I may be so bold as to offer a tad of advice re travelling in convoy...have a plan to meet up should you get separated, for example if you have "lost" each other say stop at the 1st service station of aire de repos after a certain road junction/exit. The chance of getting separated at some time over a 175 journey are, I would say, are between 25 - 40%. If you have mobile 'phones each then the problem recedes dramatically. On a positive note your kids will lovely La Garangeoire. As regards pitches, if the site will allow you have a walk around and choose your pitch carefully. Serviced pitches are nearest to the entrance/ reception/ bar etc but they are also surrounded by tall trees that can cut down on the sun at certain times, and also block satellite TV signals. Pitches on the perimeter have a paucity of water taps and drains!! get back to me if I can be of further help.