Hiya, try to see if they're able to resolve the problem between themselves, hopefully eliminating the need to tell tales and involve you in a blame game, it'll also help them to sort conflicts out amicably between themselves in the future.
Sit with them and ask the child with the grievance to tell the other what they found upsetting, or that they found was a cause for concern and consequently shared/disclosed by telling you.
Ask yourself if the concern is an appropriate or an inappropriate one - was anyone hurt physically or emotionally or is it maybe a situation that involves understanding, turn taking.
1. Help explain why appropriate behaviour is ok in your setting - why you allow things as a parent/professional that the other child may not be familiar with.
or
2. invite communication that helps each child to explain what they did and why.
Do you have any child initiated rules for expected behaviour - ones they've set themselves for everyone to treat each other fairly? It may be one way to help them govern their own and each others behaviour and responses. Call on them individually, reminding them to check for themselves if they feel a rule has been broken, & have them discuss what they agreed to and why eg: hitting hurts and isn't nice.
A time out space can also help for self elected exclusion, somewhere children can go when they want quiet and privacy - a safe haven corner maybe, where any one disrupting the quiet can be asked to take noisier play to a more suitable area.
Conflict resolution activities on partnership for children.org.uk may help with ideas.
It isn't easy, best wishes with it xx