Have a look at www.campingcard.co.uk ascu card site you can search by date, it shows there are 6 sites open in France and 6 in Holland as well as a couple in other places. You can then chose one with the facilities you would like.
Le croix de veux pont near paris is open and I believe has a pool. Good luck
I don't think that sites with indoor pools will have them open at that time of year. We've been to sites with indoor pools which still close when some other of the site facilities close down. I'm sure I've read somewhere that La Croix pool isn't open except during the summer months.
Unfortunately the 'camping season' in France and other European counties isn't as long as it is here, and they tend to open facilities only when the sites are likely to be full.
We have used the ACSI site les Gros Joncs on the I'le d'Oleron a couple of years ago, in March. The indoor pool complex was open, and beautifully warm. They are open all year, so you could call and check that the pool will be open. I think they may make an extra charge for this now, but we weren't charged.
Don't forget to leave a review of the French and other European campsites you have visited!