Whats the halacha to drinking wine/alcohol for a minor when they need to drink four cups of wine on pesach and if so then is in it going against Dina D'malchusa Dina?
I will offer a partial answer to the question. Many states have exceptions to the underage drinking law that for religious purposes it is allowed. Many states also have an exception when on private, non alcohol-selling premises, with parental consent. I got this information here: http://drinkingage.procon.org/view.resource.php?resourceID=002591.
Although this doesn't answer the halachah aspect of the question but practically the problem wont apply in many states.