It's the removal of agency, I guess. The connotations of untrustworthiness, lack of perspective and general disrespect towards children's self-recognition and self-advocacy as well.

Like... there's nothing inherently objectifying about saying "kids don't have the cognitive abilities of adults and need guidance" but the way it's used definitely doesn't imply that the solution is to get those kids involved in decisions regarding themselves to help with perspective-building.