Figure out the most effective discipline you're allowed to use and have a pow wow with them. Explain exactly what tattling is and that it's just as bad as the action they're complaining about. Explain what all the rules are again, like fighting, hitting, whatever. Tell them that the next time someone comes to you and tattles, everyone involved in the entire incident will have X happen and the item or activity in question will be taken away. Follow through. I'm not a fan of paying kids to be good with rewards, but you could also (in ADDITION to some discipline) offer some fun incentive at the end of the week (or day) if NO ONE has tattled or something, so they have a group project and attitude. If one person tattles and blows it for everyone, they'll be more careful next time.
Be as firm as you can, they are old enough to understand you and to control themselves. I would send a note or speak to each parent too and tell them you're having this issue and ask them to teach about tattling at home. Tell them it's a message for everyone so no one feels singled out.
In our house I lump tattling right in with "fighting" and warn them there will be a consequence if I have to resolve the issue. They're allowed a certain amount of light bickering and have their own toys-they don't always have to share, but if it escalates to the point of disruption, or someone tattles, I calmly tell them I will take the item away and they will both get a consequence if they don't resolve it. So far I've never had to do anything, because this works since they know I mean it. Now it's to the point where if someone tattles (rare) I say, "Are you sure you need my help with that?" and they say, "no" and run off. And they know they're not allowed to act that way out with other kids or at daycare.
My 4 yo came up to me the other week and said, "Mom, I don't mean to tattle, but (her youngest sister, our 10 month old) is chewing on the cords." Phew! My computer cord had fallen down into her reach and I didnt' know it. Then she got the," Good job, sometimes it's OK to tell on people" talk.