I doubt that Bob Ewell made much impact on Jem. Jem knows him for what he is: an alcoholic, abusive man who does not take care of his children, his home or himself. He is universally despised by most of Maycomb's citizens. If anything, Jem would lose his childhood innocence (if he hadn't already) about adults. To the children, an adult was a person to be trusted and courteous to. I'm sure Jem never encountered an adult male with no socially redeeming values until he was attacked by Bob Ewell. Ewell wanted retribution for Atticus showing the town what kind of man Ewell was while he was on the witness stand. To attack two children in the dark of night while they were returning home from the haven of a school function surely would let Jem know that not all adults are created equal.