I don't know of anywhere I'm afraid, I was just curious because I'm in Northern France too, not far from St Omer.
There may be an English playgroup in the city. Someone on a similar thread for an English playgroup in different town mentioned something called Message which puts English speaker in France in touch. I think they have a Website. I have never used them so it's not a personal recommendation.
Nothing round here but we have done 'play dates' with English speaking children that we have met.