In my opinion in sever flat foot is better to do surgical correction befor it become symptomatic. The results are worse if the pain is instalated. We do for correction sinus tarsi arthroeresis with special titanium screw. We have a serie with more than 50 patients with a minimum 3 years followup.
I totally agree with Dr Ulici. In severe cases symptomatic or not they will become symptomatic as the child gains size and wait. There are so many simple procedures that do great in children that aren't skeletally mature for flatfoot that will change the future for their feet