They must be getting "taught" (brainwashed) into the ways of the of the Others, so they won't cause problems when they grow up there on the island. since the otehrs can't give birth to children yet. The only problem I can think of with teaching those children, is that they would be taught to be smart, and to become manipulative and good with the studies of psychology, sociology etc. The problem with that is, when they get too smart, and they'd over-run the Others. So why steal the children? You tell me....