Hi,
I can't really comment on Spain or Greece as never been there but have spent quite a bit of time in France.
Most of the holiday places in France in August will be very crowded and accommodation expensive. If you are able too I would consider going outside of August, (and July). Also motorways heading south in France are clogged most Saturdays in July and the first couple of weeks in August as the French head south for their hols. If you have to go in August I would try for the last two weeks, the second week would then be cheaper and less crowded as many people will have gone back for the start of school first week in September.
I don't know what type of accommodation, area, how far you want to drive, things to do etc you are looking for but here are a few places we have enjoyed with our son, from birth to now almost 6
Ile de Re - a lovely island just off the coast at La Rochelle. Great if you like spending a bit of time of the beach, very clean sandy beaches, lots of very safe car free cycle paths all over the island (you can hire bikes very easily), nice little harbour towns with very good seafood restaurants for Mum and Dad (and good wine). The problem is in August it is very crowded and expensive as very popular with the French, including a few film stars etc.
Loire Valley - perhaps not the first place that comes to mind with children but we spent 2 weeks there when our son was 3 and a half and had a great holiday. Lots of things for kids to do and see and for Mum and Dad, especially if the chateaux are of interest. There is a good range of accommodation at all prices, and if it has to be August not as crowded as the coastal resorts
Nice area - we spent a week there in June last year, nice beaches (although not all good for very young children), lots of things to do e.g. acquaparks. Again though, very packed and pricey in August. Also a long drive from the UK but there is a very good public transport system there and you could fly as you don't really need a car if staying in the town centre.
French Alps - we have spent a few holidays there, including Annecy and Evian les Bains. I would say with children there is more to do in the Evian area, including some trips to a few interesting places in Switzerland. A good place if you like an outdoor type of holiday, swimming, going up mountains (my son loved seeing the snow but where we live it is a rare thing - perhaps not the same for you). The downside is the weather can be bad sometimes even in the summer so you need to check if there are enough indoor things to do. However it is less crowded and expensive than the coast.
If you decide on France a few places to look for self catering accommodation are
www.homelidays.co.uk/
www.holiday-rentals.co.uk/
www.abritel.fr/
www.gites-de-france.com/
We have used all of these sites and always been very happy with the places we have booked.
A great site for things to do for children, where you can search by age, town, region of France is
www.familiscope.fr/
Hope this helps as a start for France
Alison