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Johnny is a sensitive boy. He cares for others, especially those that are helpless like the children. This is perhaps because he has felt so helpless in his own childhood. It is also probable their cigarettes started the fire.
I think Johnny wasnt scared because all he was thinking that he started the fire and he had to do someting about it, like get those kids out no matter what, because if something happened to them in the church and it burned down. Knowing kids die partially because of you, he wouldnt be able to live with the guilt.