They have seen other children baptized. Friends in other religious groups have been baptized. They come from a family where everyone has been baptized for generations. They have heard that God wants us to be baptized, and they want obey God. I call your attention to some additional reasons that, in my judgment, are significant factors.
Many Christian homes are either deeply troubled, very unstable, or broken.
Many of our children are in insecure environments. Between troubled homes, separated homes, broken homes, one parent homes, homes in which there is abuse, and homes that are too busy to nurture and care, many of our children crave adult attention. One of the few things a child can do to secure immediate, positive adult attention is be baptized. One of the things that a child can do to seek security is to be baptized in insecure circumstances. Children are responding to an oversimplified concept of obedience.