I think it's partly just about getting the message out. Many people who have already gone through child marriages have had horrible experiences. People there can tell what's wrong with child marriages because they know from having lived them. I think that's one very useful form of information.
Where there are laws, they should be enforced. Where there are no laws, they should be enacted. Simple advocacy for change in the laws to prevent child marriage or to enforce the laws where they do exist but they're not enforced helps not only in a legal sense, but also as a form of public education because advocacy about the laws and their enforcement tells people what's wrong with these marriages.