Date: Probably 1980-2005.
Country of origin: Probably the U.S.
Two tenish siblings, a boy and a girl are living with their abusive parents. One day they run from them during a shopping trip to the supermarket (?).
The father is after them, yelling, and they hide in the back seat of an empty car in the parking lot.
Shortly after, the owners of the car arrive and they drive away with the kids inside but they don't notice them.
They are discovered later and it turns out that these involuntary kidnappers are nutcases.
They mistreat the kids, they lock them in a closet, put weird clothes on them, take photographs of them.
There is a similar "kidnapping" scene in Running Scared (2006) but it's not it.