Hmm not sure what you mean. The hashes are shipped with iOS, and then (if parental controls are enabled) compared with the hashes computed against the unencrypted data on-device, and displays a warning to the user and their parents.
I'm not sure how the image is displayed on the parents' device - it could be sent from the child's device, or the image used to compute the hash that the child's image matched could be displayed.
I'm not sure how the image is displayed on the parents' device - it could be sent from the child's device, or the image used to compute the hash that the child's image matched could be displayed.