Want to open children's photography, want to buy a full-frame camera, and seek a cost-effective machine.

I recommend Canon 6D, the cheapest Quan Huafu, which is worth starting with. The noise control ability of the whole picture is much better than the afterimage, and the clarity and color depth of the image will be better. Canon's machine lenses are more and cheaper, the whole frame is better than half frame, and the focal length is better controlled. For example, if the focal length is 50mm f 1.8, the focal length at half width will be 60% longer. ...