France is such a huge country! Where do you want to go? North, South, East or West, seaside, or inland? Which ferry port are you travelling from, and how far do you want to drive?
Is this your first time in France?
Firstly I'd suggest that you decide which area of France you'd like, based on the journey times, ferry ports, etc., as above.
Secondly, I'd suggest you have a look at the Eurocamp, Keycamp and Canvas Holidays brochures, because all these companies feature sites which have clubs for particular age group children. This means that if you choose one of those sites, and take your own equipment, there are likely to be lots of other children of similar ages.
Thirdly, once you've decided on an area ask for advice about specific sites.
Outside of peak season (and Easter is NOT peak season in France, indeed many sites won't be open) then you don't even need to book! I know this seems hard for us Brits to understand, but in Europe there's a much more relaxed attitude to pre-booking, except in the high-season summer holiday. Many caravanners (us included) don't book at all, and just go where the weather, or the mood takes us. Have a look at the websites of www.campingcheque.co.uk, www.touringcheque.co.uk, and www.campingcard.co.uk - all these offer discounts over normal site tariffs and specialise in low season holiday, so will offer plenty of sites open at Easter.