I think, you first need to check with the doctor whether this is a flat foot or not, and even if it is then also there is no need to worry as some degree of flat foot decreases with age. Say, by the time kid turns 7 or 8. And if the kid has no symptoms attached to flat feet, should be left alone. Also, for flat footed children, all one requires is, have simple footwear, a well-fitted, supportive shoe with built-in arch support is generally the best.
I suggest, talk to the doctor or podiatrist and let him assess the child's feet and advice as what should be the course of action. To know more on the condition, I came across a few links that you may want to read. Hope they help.
http://www.abc.net.au/health/talkinghealth/factbuster/stories/2009/08/13/2654510.htm
https://www.betterhealth.vic.gov.au/health/healthyliving/feet-children