We've also stayed at Des Ormes, which is linked above. Amazing pool area, though vast, which may cause you stress if they try to go in three different directions at once.
We have also been to La Croix du Vieux Pont, which was also mentioned further up. We've actually been 5 times, starting when the DCs were 3 and 4 (so similar ages to yours).
Pool complex is nothing like Des Ormes, but I'd say better for little ones with a large toddler pool, complete with their own slides, and a splashpad.
It also has a man made beach and lagoon on site, which is lovely. We've spent many hours building sandcastles there.
There's also a sort of activity area there, which has always been a hit with mine. There's ride on quad bikes (perfect for your 3 - and 5 year olds), a bouncy castle, trampolines, and bungees. You can pay about €70 to get a card worth €100 and you just have it stamped each time.
There's also playgrounds dotted all over the parc, the biggest of which was upgraded a couple of years ago and is really lovely now. There's an indoor
www.eurocamp.co.uk/campsites/paris/pa012-la-croix-du-vieux-pont/ataglance.html
It's about a 3 hour drive from Calais, and it's all motorways until the last half an hour or so, so a really easy drive.
We've stayed in every form of accommodation on that site, excluding the Safari Tents. From the 'Classic' (cheapest) caravans to the Lodges. Our favourite was the Aspect range (the top end one). We actually preferred that to the Lodge.
All the caravan accommodation comes with a retractable baby gate on the decking, so you can open the doors and let them roam freely, without worrying they'll wander off.