In To Kill A Mockingbird, Jem and Scout were disappointed in Atticus because he wasn't athletic. What changes their minds?
Scout and Jem find new respect for their father when he kills a rabid dog with a single shot.
Being children, Scout and Jem are quick to notice that their father isn't like other parents around town. He's older, for one, and despite his wisdom and demeanor he doesn't seem to have any enviable abilities, hobbies or possessions. This may seem ungrateful and petty on their part, but it is entirely understandable; Scout and Jem are looking for ways to keep their father "competitive" with other parents, at least in their own minds, because they want to be proud of him but lack the maturity and worldview that values Atticus's wisdom over his ability to throw a baseball. This is brought into focus when other children mock them for Atticus's choice to defend Tom Robinson, making him a "nigger-lover". They are also confused as to why this doesn't seem to bother Atticus as much as it bothers them.
Their faith in Atticus is restored when a rabid dog happens to wander into the neighborhood, and rather than kill it himself, the sheriff asks Atticus to do it. Atticus effortlessly kills the dog in one shot, from a considerable distance. It is revealed that in his youth, Atticus was the best shot in the county. Atticus's lesson is, in short, that one should never reveal their talents or put on a show simply because someone demands it of them.