Resting the camera on the dashboard of the car to keep it relatively stable would be a good approach. Then it's just the car's movement and the cars outside that will create the blur and not camera shake. This will have to be experimental as far as shutter speeds and exposure are concerned. Obviously night time would work best if I am going to record light streaks. The camera pointing at back seat and kids with blur outside?
This Mulled Blackberry Wine is too good to save just for Christmas, I've served it on Bonfire Night whilst waiting for fireworks and it was SO good! Toasted vegan marshmallows and sparklers obligatory!