First of all, make sure you're correct that it's flat feet (pes planus). If so, don't worry about it. This is normal and common in kids, especially at this age. The arches don't start forming until age 3 and later. If this doesn't self-resolve years from now, try orthotics, see a podiatrist, bla bla bla. For now, don't sweat it, and enjoy your child.