Because only stabbing or sneaking was ever covered by the books, IIRC. Overcoming mean defeat, not roleplaying. Through I guess that means that the child would get experience for beating up other children, but the system was VERY lackluster (if not outright non-existent) for most other methods which were often house rules. OoTS DOES have several house rules, so maybe Grandpa got experience that way. But the characters often talk bout killing monsters for experience, implying it's probably the best method to parody the system.
Also, children probably shouldn't be encounters typically, but I don't think that's a rule anywhere.