Child marriages are an evil because it causes different problems in a child's life. Some of the problems are ;
If a child is married, the husband child have to take up the whole property and the house of his father. And it is very difficult for him to manage all his father's property and it would all go into someone other's hand.
In the case of child marriages, most children are not completely educated but still they marry.
Some child even leave their husband or wife and run far away from him/her.
They have many high high demands from their family members which they have to fulfill, like giving birth to a baby, doing a job and earning money etc.
There are many more other problems.
I hope this answer might be helpful!