Sorry, I am still confused.
The so called punchline tells us that because the family had a lot of children, the blind man did not get on the bus. I do not see how the two things are related. If the bus was full then nobody was getting on it and it is not the families fault, if there were spaces then the blind man would have got on it. The only other scenario is that part of the large family did get on the bus but this is specifically excluded earlier in the joke when it is written that "the large family find themselves walking".