After reading all these postings, I bought 2 pack of 'Hernidex' , it is really cheap(about $5 per pack) but it is no comparison with 'Staedtler' made in Germany 的黑黃間三角鉛筆, not even close! As one mom's comment:墨色好似唔夠深, 寫字無力會寫得唔靚。
Staedtler is much more expensive, about $38 per pack. But I think it worth the money because my son's hand is not so strong, it's less tiring for him to use it.