Is it OK for parents to be supportive to children's negative emotions? New research suggests that whereas mothers who are more supportive of their children's negative emotions rate their children as being more socially skilled, these same children appear less socially adjusted when rated by teachers. Source 6g.
Wow this really depends. There is not right or wrong answer but in any case the negative should be turned into a positive. For instance expressing frustration about being bullied you might say "I know how you feel, its OK to be angry because what they're doing is not nice, but don't keep the anger, let it go. Let's find something for you to do (Activity)."