Child marriage is indeed one of the most dangerous, inhumane and primitive behavior which modern societies due to the advancement of science and morality counts as a heinous act.
If we step outside of our comfort zone and enter the less fortunate areas in the world, where people do not have governmental systems to protect them or their minimum human rights, then we realize religious authorities become more dangerous to people than in modern societies. In societies like that believers still could behave as was demanded or suggested in the holly books. Few examples of that is slavery, oppression of women, stoning, witch burning and child marriage. In such areas under the influence of religious oppression change is much harder, especially where behavior of the role models advocates the same behavior we like to see it change. In the case of child marriage Muhammad the prophet of Islam married Aisha one of his wives when she was six and consummated at nine. The problem in this particular case will always be the creator of Islam doing something anyone who is literal about Islam would be allowed to do by their belief system even if it would be illegal by law.
The only reason we as humans are constantly advancing in our standards of living is our advancements in technology and science due to skepticism, freethinking and imagination.We have to realize the same way as we do not behave any longer as the holly books demands even as believers, we can and should be skeptic about everything we do and constantly look for better answers through science, the only accurate and worthy tool for understanding of our universe.