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?
Remnants by Greg Sand is a series that i find quite creepy but at the same time moving. He weaves together 3 images of a person from different points in there life to create a portrait of a person who has passed on. When you look at the images you can see flashes of youth in parts of the portrait but in others u see a tired frail person. I think this idea is extremely strange but i do like it.