Well, with models like SD at least, the datasets are large enough and the employees are few enough that it is impossible to have a human filter every image. They scrape them from the web and try to filter with AI, but there is still a chance of bad images getting through. This is why most companies install filters after the model as well as in the training process.
You make it sound like it is so easy to even find such content on the www. The point is, they do not need to be trained on such material. They are trained on regular kids, so they know their sizes, faces, etc. They’re trained on nude bodies, so they also know how hairless genitals or flat chests look like. You don’t need to specifically train a model on nude children to generate nude children.
