@Shingwauk I think the point of the ending, or at least my interpretation, is that
on the bus, it seemed like the mom might have panicked and abandoned the kids and the dad. Her initial reaction is to justify this to her family and the girlfriend, blaming the driver, but if she later realizes what she did then maybe she could better sympathize with the dad, and that might be what brings them back together (speculation). The cigarette, on the other hand, felt like a little vindication moment for the dad, who I think recognizes what just happened. His son asks him something like, "Do you smoke?" and he replies "Yes." Even though he doesn't seem to be a regular smoker, I think that particular response indicates that he has taken ownership of his actions and himself, and that he is trying to become more comfortable with his identity/sense of self and assert accountability for his own actions, which is clearly something he has been struggling with throughout the film.
Also, I just found it amusing that both of you guys tried spoiler tagging, but neither of the tags are in the right place--at least you had good intentions