Marking someone's body with a marker is an assault when the person marked doesn't consent to it. Assault doesn't necessarily have to be hitting or kicking a person, it can be simply touching someone in an offensive manner (such as shoving or grabbing someone). In this case, touching the boy with a marker and marking his body can be construed as being touched in an offensive manner.
A person can be kidnapped if he's held against his will and deadly force is used or threatened. There's also a lesser crime called unlawful restraint. Unlawful restraint occurs when someone is simply held against his will (regardless of the fact that deadly force or the threat of deadly force was not used). In this case, I don't think it's a stretch to assume that the boy was held against his will, but we can't tell from the limited facts provided whether deadly force was used or threatened.