Can a child that has not been immuniised harm children who have been imunised?
Chickenpox can still be contracted by people who have been vaccinated. And they're contagious, too. So they could then pass it on to someone who is more at risk for severe complications.
Not vaccinating definitely provides a vector, so it is a risk factor both for the child that is not immunized and for anyone to whom the child might spread disease.
And you can't simply assume that other kids are getting immunized. Modern society is too mobile, for one. Kids come from all over the world, and maybe from places where polio, or some other crippling disease, is much more prevalent. And if one person is thinking about not getting vaccines, then you can bet that other people are thinking about not getting immunized.