"The children are walking behind their parents."
Translation:Die Kinder gehen hinter ihren Eltern.
For Akkusativ, there needs to be movement from one place to another. However, a place can be a mental area in which you can move around. You could image that the children are in a box-like area "hinter" (behind) their parents. While they keep walking behind their parents, they stay in that imaginary box which is why you use Dativ here.