One of my friend's son is taking 畫畫 lesson. 家長可以陪, but it is one-to-one. My friend said the teacher is very very good. She had him going to a class before but she didn't like it - she wasn't sure whether the paintings were really done by her son.
I can get you more details if you want. I think it is in the Kowloon Tong area.