Wise words Mazza, I think also in your case, children only really let out their emotions to the people they are closest to, so for example if they're finding something upsetting at school, their teachers'll say they are "fine" at school, but as soon as they're with someone they feel safe with they'll start lashing out and the emotions come out, sometimes in nasty ways, but you have to look beyond that and try to figure out what's making them emotional and see the lashing out as emotional deflation IYKWIM, and what they need is just for you to be consistant and reassuring. And feel on some level glad that they have someone like you who they feel secure enough with to let their rawest emotions out with… over time this'll become more refined and they'll eventually be able to tell you their problems in a more rational way
That's not to say you should allow rudeness or hitting etc.. but while letting them know it's not okay, try to look beyond it to what is causing the build up of stress that is being let out when they're around their "safe" person.