Re-entering Hong Kong with Smart ID

I think you should contact HK Immigration Department to get the authentic answer.
http://www.immd.gov.hk/en/contact-us.html

Hi, yes, please bring along your Juvenile ID and your passport. When you are in HK, go to the immigration in Wanchai to apply. Or you may also make an appointment on the hkimmigration website, so that you don't have to line up when you there.

Go to: immi.gov|inquiry,hk to enquire.

Thanks for asking.