Ah, the joys of having children who are very excited in trying to figure out the justice of their world. My question to you is this: WHAT exactly happens when your children come up to you to tell on each other?
Most of us mean well, but we talk too much with our kids. That is, the customary response to tattling children is to engage, either with questions drawing out the particulars of the incident, or to explain to our kids why we don't like tattling, or some other exchange. In the end, our kids are encouraged to tattle because it is a stunning way to engage us. We see this also in how excited they become when they are rushing up to "tell" on each other.
My strongest suggestion would be to read "How to Talk so Kids Will Listen, and How to Listen So Kids Will Talk" by Faber and Mazlish. This is probably the most valuable book I've ever read in regard to verbal exchanges between people.
Then, here is a suggestion that you could use right away. The first is to put the onus of action on the tattling child. Example:
Child: "Mom, Kate's pushed me!"
Mom: "Hmm. Kate pushed you."(impassive, just stating a fact)
Child: "Kate's pushing me and I don't like it."
Mom: "You don't like it when Kate pushes you."
Child: "I hate it."
Mom:"You are really mad because Kate pushed you."
Child:"I'm gonna tell Kate not to push me anymore."
At this point, the child has talked their own way through the problem without the parent taking sides or solving it for her. If the conflict continues, we continue listening and rephrasing. Sometimes our kids can come up with ideas to resolve it on their own. This is the hard part: their decisions may not be exactly what we are comfortable with, but unless they are unsafe, it's worth it to give them a try. At other times, children may need an adult to engage them in some problem solving by making a list of everyone's ideas (writing them all down without criticism) and letting the kids choose one they both agree to try. Give them a few chances to figure out how they might like to work things out; this actually strengthens the executive function skills that are necessary for healthy social interaction and multitasking.
My short bit here is no substitute for the book itself, but I hope you get a chance to see that this is a wonderful opportunity for you to step back and not be put in the position of referee between your kids, which ends up being incredibly draining by the end of the day...all of the enforcing punishments and "Mind your own business", etc. Our eventual goal as parents is to make our jobs a little easier as our kids come into their own capabilities, and to empower them to solve their own problems when it's appropriate. The techniques in this book do just that, and I hope you have a chance to pick it up. It's been an unbelievable help for me in my work with kids.