We do a mix of holidays each year. Usually one or two CP/Eurocamp type self catering holidays during half terms then a week AI during the summer. Both very enjoyable in their different ways.
If you do consider center parcs, have a look at Erperheide. We had a 0.5, 2.5 and 4.5 when we first went and found loads to do to fill the week. There's a great swimming pool with loads for young children, horse riding, jeep safari, big indoor soft play, kids craft activities, mini disco, little farm you can feed and stroke the animals, play areas dotted around. It's not too big so fine for little legs, or take a scooter or pull along wagon. It's a bit of a drive at 3.5+ hours from Calais, but worth it.
De Haan is another good one, only 1.5 hours from Calais and also great for younger children. It's near the beach with big sand dunes, huge soft play, farm, jeeps. Pool isn't quite so good but still has a toddler pool with slide. Can do a day trip into Bruges or hire bikes and cycle along the coast.
Many Eurocamp sites are also excellent with good pools and kids clubs. You have a good budget so could afford nicer accommodation and eating out each day if you don't feel like cooking.
I don't have a particular hotel recommendation for AI as the ones we've been to haven't had loads to do nearby so might not suit you, but have a look at Zafiro hotels in Majorca/Menorca, Royal Son Bou in Menorca, H10 hotels in Spain/Canaries. Most have family rooms with a seperate bedroom and living area. I would never have thought we'd do an AI holiday before kids, but they are just so easy and relaxing you really can spend quality time with them. No worrying about what's for dinner and getting a food shop in, no constant moaning "I'm hungrrryyy", no planning what to do each day to keep them occupied. Kids club, pool with slides, ice cream on tap, mini disco, everyone's happy!