This is great! Robin Unger found this image in which there are two women holding small children, and he even admits that in each case, the child is sitting on the mother's hip. It's not actually the hip; it's the pelvic bone.
In each case, the child is sitting on the mother, and the mother's arm is securing the child- keeping it from toppling over. The child's weight is being added to the mother's weight, and her arm is serving as a strap.
But in the Altgens photo, that boy is definitely not sitting on his mother's pelvic bone.
Here are all three at once: