From the SCMP article that ran on or before October 1st under the title "Hunt for knifeman after three youngsters stabbed":

Two of the injured - a 16-year-old girl and an 18-year-old man - were in stable condition in Tuen Mun Hospital yesterday. The youngest victim, an 11-year-old boy who suffered cuts to his forehead and stomach, was discharged after treatment.
A Chinese-language article, complete with a sketch of the assailant, was posted on Oct 3rd.

The SCMP article says that he was wearing a t-shirt, shorts, and a black backpack. No description of the footwear. The oldest victim said that the guy approached him with some glowsticks, said "What's up?" and commenced stabbing.