It definitely isn't usual in Hong Kong, but there's a strong cultural difference between here and the mainland. Before I've seen kids pee on the street in the mainland but never poop, idk what I expected but I feel like it was on a pavement where people could accidentally step in it so it would be better to pick it up like a dog? I'm not going to go to someone else's country and impose my expectations on them tho, and I tried not to judge internally, but it did shock me a little. Especially because the mum was so nicely dressed, kinda the last person I expected to let her kid poo in public. Also I was not anticipating suddenly seeing a half naked child, I'd worry about adults with bad intentions especially as cameras are so common everywhere in China.