Public transport in Hong Kong depends not on the height of children, but on their age. Children octopus should be used when they are 3 years old, and adults octopus 12 years old should be used. That is, children aged 3- 1 1 can use children's octopus, and the cost is half that of adults. You don't have to pay half price in cash with Octopus. Minibuses are the same size and price regardless of adults and children. Take your certificate with you when you go out, in case you check your age.