Woods road in Wanchai would be the place to go, there are several bike shops there. I would advise against training wheels, the best (and safest) way to teach your child how to ride is to get the shop to remove the pedals and lower the seat, let the kid use it as a hobby horse for a while until he/she gets used to the brakes and steering. When you feel the child is ready, put the pedals back on.