Generally a son complains that they don't get the chance to do an activity, event, etc. Now you got one is the parents way of making the kids do something that the parents want them to do. Example: A kid steals from his friend. The friend says that he'll never talk to you again. A week later the friend calls and says he's sorry, and he'll give you another chance. Now you have the chance to be friends again, as long as you don't steal.
It means `` Be careful what you wish for``. Many children and adults alike complain that they do not have the ability r the chance to do something and when they do get a chance they are unable to perform or realise that it is not what they really wanted.