Question

why has the poet brought an the image of merry children "spilling out of their home"

Answer 1

I understand that you are a class XII student. I am too and I can only explain on the basis of what I believe.
"spilling out" refers to the flow of a liquid which symbolises rush and energy of life. The poet uses this phrase to contrast between the pale, dark and decaying atmosphere which is created inside the car and the rush, the energy, the flow of life outside it.
Hope I've explained it well.