Normal feet in young kid is flat until 5 years old. If it is still flat, then need special arch support placing in shoes. Before that, just normal shoes with support is enough. Besides, exercise can help more than just that arch support. U can do some tip toe exercise, jumping, toe exercise as well. Flat foot is caused by weak muscles that support the arch. Therefore, exercise is more important to get it improved. Some doctor got no such knowledge as well. Ask a paediatric physiotherapist la. Furthermore, flat foot may cause back pain or knee pain in adulthood but not a must. It depends on body weight and what activity u always do. E.g. More pain if u go running without running shoes. Therefore, buying a pair of shoes with arch support for your kids in early stage is enough. Do more exercise after 3 years old is important to keep muscles strong to keep that arch. No need to waste money to buy so many stuff
My kid is now 3 years old and we suspect he has the problem, however, dr. Kong is not our choice as it is more business like. You may try paediatric physiotherapist. As mentioned by shuna, strength the muscles is the way to improve the 扁平足. you may also check 寶血醫院, they have the assessment service.