I used to say I don't speak whinge and I can't understand you when talk like that.

I also found questions back helped. Losing my cool seemed to feed it. So often I would grit my teeth lol.

I think asking questions helps them to formulate their own thought processes when they aren't happy about something.

Ask them why they think they can do if xyz is annoying them. Go through the tools they would have learnt at kinder ie how to deal with conflict. Ask them what they think you would do. And if they are still losing the plot ask them if they need some quiet time to calm down so they can think about it more clearly. Express understanding towards whatever emotions they are feeling. This is a well known negotiator skill even with over emotional adults. and if its something like having friends over talk about when a good time is to organise that. Sit down and work out when they can have a friend over etc. think about it from their perspective, if you wanted to do something and realised you couldn't you would have your own inner dialogue about how and when that could happen or what you should do. This is something learned by life experiences and doesn't come to them naturally. So that's the tools they haven't learnt and that you can teach them