They're already doing this. Why do you think they want perinatal abortion laws? Pediatric organs can't be transplanted to an adult unless they are about the same size. The vessel sizes have to match for anastomosis, so babies can't donate to an adult. In the novel "Ender's shadow" by Orson Scott Card there is a hidden industry that raises children strictly as organ banks. Not the best sci-fi novel ever but there's some interesting dystopic concepts in it.