Hi Maichiang, I was worried about the same thing too for my son when he was about that age. Our doctor said all babies start out flat footed so when they are a bit older you can have them checked out...
You probably could buy those good quality shoes or sandals that have in-built support. Clarks is very good for this, my mother insisted on buying Clarks when we were kids because both me and my sis have flat feet, so she buys for my son too. My dad has flat feet too... we all walk, run and do everything reasonably well. The only complaint is that our feet can get a bit sore after walking or standing for too long....other than that we function like normal
My sis had some special soles made when she was younger....I guess they helped.
My cousin has really bad flat feet that create suction noises when she walks on tiles...she seems fine though, has 3 kids of her own and babywears each one constantly.