This kind of pasta is called "alfabeto", hence ABC粉, and the Italians used it to thicken their soup. Because of its tiny size, it's a good choice when babies start to eat solids. If you can't find "alfabeto", you can use "risoni", which is shaped like rice, or "stellette", which is shaped like stars. As your child gets older, you may even change to the dinosaur shaped pasta to entice him to eat.
My son started to have pasta when he was about 10 months old.
I don't live in HK so I'm not sure if Welcome or Park-In Shop will have them. Try CitySuper or Great, all those upscale supermarket.