Approach them at their eye level and depending on the situation or severity allow them to see how their friend is feeling for instance of feeling sad "oh no it looks like our friend doesn't like when you ___" or "hands are not for hitting" and encourage the friend to say "please stop doing that, I don't like that" while putting a hand out as if saying stop. If the child is constantly hitting and it is severely intense that one must stop asap as a teacher who can't say "No" but explain why not, I would say "stop your body friend." What you are doing is scarying me." It makes me sad to see you hurting my friend." Eventually child keeps hitting seperate the children and later have a conversation with parent to find out how to better work with the child at home and make plans in the classroom such as set goals etc to share and/or help with turn taking