I can heartily recommend Domaine du Château de Drancourt near St. Vallery-sur-Somme.
Nice and cheap if you avoid French school holidays, has 3 nice pools on site with nice quiet roads nearby for a pootle by bike.
Yes, thoroughly enjoyed a week in May there, last year. The riding isn’t very interesting but depends what you’re looking for.
It’s a quiet, smallish campsite with little entertainment. (Which suits my family.) It’s close to the Somme, Amiens, an hour from Calais, an hour from Boulogne which has a great aquarium. I rode out to the battlefield at Crecy, through the forest. St Valery housed Joan of Arc and has some great places selling moules frites and is very pretty. Recommended!
Oh and it cost less than £500 for 5 of us, including ferry crossing and EU breakdwon cover.