Christianity doesn't talk about child marriage. Rather, Christianity talks about love. In all but severe cases, with extremely messed up conditions, child marriage is not love. Love has to do with waiting for the child to become an adult, and then guiding him or her to make their own decision based on both love and wisdom.
In Islamic states, child marriage can be a protection for a child where there is no other way to protect the child. For example. A man who is a master and owner of property and slaves, might fall in love with a slave woman and create a female child by her. Because marrying the slave woman might endanger his estate, and both of their lives, neither the woman nor the master would want to get married. So, to protect the female child that is his daughter secretly, but is a slave by birth outside of marriage, the master marries her... but would never even think of having sex with her.