Here is the basic way I handle tattling, when a child tries to involve me in a problem she ought to handle on her own--I think of it as a tennis match. Let's take an example, and I'll show you.
Morgan: "MOM!!!! Ryan just threatened to cover me in snot balls!!!!!!" (the serve)
Me: "Well, what did you say to Ryan about that?" (the return)
Morgan: "Well I said 'I don't LIKE that, Ryan!' but he won't stop!" ('He won't stop' is a return to me, a request for me to do something rather than her. Fortunately this is an easy lob to hit as we're all in the car and I'm a direct witness to the events.)
Me: "It doesn't sound to me like he is continuing with the threats. Sounds like you handled it." (right back at her)
Morgan: "But MOM!" (her ball goes wild)
Me: "It sounds like you're still mad and that's okay. But you told Ryan how you felt and he stopped, so you handled this problem all by yourself." (Game, set, match!)
Basic premise: keep the ball (their problem with someone else) in their court. Consider this kind of tattling as an opportunity to hone problem-solving skills. Otherwise they'll learn to look for you to solve their problems, and I don't know about you, but I'm already busy enough solving my own problems.
I am playing Tattling Tennis a bit with Sean lately, and it occurred to me that I do it a bit differently with him, because he is three and still learning the problem-solving game. Today, he tattled on Ryan in the car S: "MOM! Ryan said that blahblahmumbleuntelligible is PLOPPY!"
FYI: ploppy is considered a deadly insult at our house for reasons still unknown to me.
Now if something like this had come from Morgan, I'd have asked her what I did in the scenario above: "What did you say to Ryan about that?"
But Sean doesn't know the rules yet, so I said: "Did you tell Ryan that you don't like it when he calls things ploppy?"
And Sean said: "Ryan I don't LIKE it when you call things ploppy." He'd have been hard-pressed to come up with those words in the heat of the moment, so by offering specific words to him as well as redirecting him back to the person he had a problem with, I gave him two kinds of help at the same time.
And Ryan said: "Okay!" and prepared to go on his merry. I intervened here and said to him "Could you tell Sean that you are not going to call his things ploppy anymore?" Which he did.
I needed Sean to hear Ryan say that so that he felt understood, and I needed Ryan to say it so that we all knew that he understood what was expected. Because he's the kind of kid who might later claim he didn't understand all of the implications of what Sean was asking. (Definitely a "letter of the law" kind of guy, that one.)
Sean must be getting the hang of things though, because he seemed to understand that he was to repeat after me when I gave him those original words to say. And soon, all I'll have to do is say to him "What did you say to Ryan about that?" Woohoo!