Nonsense. This is just garden variety exploitation, and the Christian children are more popular targets, since Christians (whose faith bears little resemblance to yours) are a minority in these countries, and thus the kidnapper/trade slavers encounter much less social, political, and legal backlash if they take Christian children. They enslave plenty of Muslim and other children, too. Anything for a buck. They couldn't care less what these kids believe or don't believe. Heck, they teach their own kids to blow themselves up.
I respectfully disagree with you if the subject is limited to slavery. If the subject is the overall treatment of children in Muslim countries and the Jihadist propoganda they take in with their mother's milk I agree with you.