The short answer to this is that Sam and Bill shoud worry about how long Mr. Dorset can hold his son because they should be afraid that the son will catch up to him. He has done such terrible things to them that they never want to see him again.
In this story, Sam and Bill kidnap the boy to get his father to pay a ransom. The problem is that "Red Chief" (Johnny Dorset) makes their lives miserable. He abuses them in various ways while they try to get his father to pay a ransom for him. Johnny thinks that it is great fun getting to be with Sam and Bill and getting to do all these things to them. Finally, the two men have had enough and they agree to pay Mr. Dorset $250 to take Johnny back.
They want Mr. Dorset to hold on to Johnny long enough so that they can get far enough away. They don't want Johnny to find them. They should feel this way because he has hurt them both (especially Bill) so much that they should want to get as far away as possible.