It is actually very unusual for an infant to have structural flat feet even though it often appears that they have.  The flattened appearance of infant's feet can be due to the following:

1.       Up to the age of approximately 3 years there is a fat pad located in the arch and this can often mask the arch shape.

2.       The joints of a child's foot are incredibly mobile and its supportive musculature is weak, so if the body is heavy, the foot will tend to collapse to some degree; so long as it is not extreme, this is normal.

3.       There may be some structural issues in the legs or feet themselves where the body will use the large mobility to compensate. 

When do I seek advice?

If the condition is normal and is just due to the first or second point above, you will note that the arch will be present when the infant or child is not taking any weight on their feet but becomes flat only when they are weight bearing.  If you note that the arch is always flat whether they are standing or not, an assessment from an appropriately experienced podiatrist should be sought.  Also if your child regularly trips over, has an awkward gait or complains when walking for more than a slight distance, an assessment would be appropriate.