The saying goes, nobody likes a tattle tail. I think she needs to understand she should tell ONLY when it's something the adult would want to know, not just little unflattering things about the other kids.
Before she tells, she should have to decide yes or no, is this something the adults WANT to know. Will this be greeted with "oh THANK you for telling me!" Playing with matches, a serious injury, a child has taken their clothes off, etc.
And she should get no reward AT ALL for telling on little catty things that no one cares to know about - the adult shouldn't even check it out, and in fact should dismiss her kind of harshly "I don't want to hear this, if you don't like them don't play with them" kind of thing.
At this age I wouldn't worry about it. Someday she may tell you something that's really important. Be happy that she feels she can confide in you. If the kids are behaving, nobody should care that she tells you everything. Do your best to keep this line of communication open with her.