What is the name of western comedy movie where a little boy kidnapped for ransom but so mean kidnappers end up paying family to take him back?

The movie you're describing is "The Shaggy Dog" (1959).

While it's not strictly a Western, it does have a strong Western setting and features the classic "kidnapping gone wrong" scenario. In the movie, a family is paid to take their son back after he's kidnapped because the kidnappers find him too much of a handful!