Hi, Here are the contact details for a sailing shop thqat I've seen children's life jackets in. Give them a call to see if they have the right size.
Sky International
Phone: 2827 5876
Fax: 2827 7120
Shop 107 China Resources Bldg, 26 Harbour Road, Wanchai
I've never been to it but HKYC have a sailing gear shop so they might be able to help as well.
Good luck