It's a slang term normally used by children which means "cool" or "awesome" :)

You are using the wrong dictionary for slang. Try urbandictionary dot com. It is like wikipedia, crowd-sourced definitions so they are very current (US English slang mostly). Unlike old-style dictionaries written by 'authorities' that only get new words if the 'experts' vote for it. First definition of wicked is 'New England slang that adds emphasis', which is correct. I lived in New England, and it is a regional word, very commonly used there, other places much less. Here are some current synonyms for 'cool' from urbandictionary: awesome amazing sweet nice hot great rad sick chill crazy tight.

The standard dictionary meaning of 'wicked' is bad or evil. However - as Adrian explained - for the last decade or more it has been used as a young person's slang word for great/cool/awesome.

This term is used for people who have a bad moral. It's not a synonym of "evil", but it's related. For example, hellcats are wicked.