At the beginning of the story, Sam says that "It looked like a good thing, but wait till I tell you." So, initially, their plan seemed like a winner. Describing this initial state of confidence, Sam says that the town, Summit, seemed to contain "undeleterious inhabitants." In other words, they seemed harmless and simple. Sam is confident that they will not have a problem.
They also determine that in semi-rural towns, "philoprogenitiveness" is strong. This means that in small towns, people tend to love ("philos") their children ("progeny"). In other words, in small towns, people are simple, family-oriented folks and therefore easy marks for a scam.
Sam goes on. "So it looked good." Then he describes the man whose child they intend to kidnap. He adds, "But wait till I tell you." When they take Johnny, he puts up a fight and Bill is injured. While Johnny (Red Chief) is their captive, it seems more like they are Johnny's captives. Johnny deals out most of the abuse to Bill. Their kidnapping plan, which seemed to have been a slam dunk, turns out to be a nightmare because Johnny is so full of energy and violent behavior (especially with Bill). At one point, Bill actually lets him go. But Johnny returns because, to him, this kidnapping is nothing more than a fun excursion filled with camping out and playing cowboys and Indians.
Ironically, when they still expect to get a ransom for Johnny, they sign the ransom note "Two Desperate Men." At this point, they are still relatively confident in getting that ransom. But by the end, they really are desperate. In fact, they are so desperate that they accept Ebenezer's counter-offer in which they pay him to take his own kid back. Their progression (Sam's and Bill's) can be described as confident to desperate.