I bought a 18k locket (photo) pendant last year for my boys grandmother. I live in Tai Po and went around local jewellery stores. No-body had any on display but a couple of stores had them out the back or could get them in (no obligation to buy). It is worth just asking around at stores.
there's also smallprints hong kong. It's not photo jewelry but your childs fingerprint in silver w/ their name. They do pendants and chains, keyrings, cufflinks, christmas ornaments etc http://www.smallp.hk/products_fingerprint.php
One of the big jewellers in HK does it. I'll post again if I come across their name, but they were advertising photo jewellery of pets a while back. So if they can do a photo of your pet, then they can also do a photo of a child.