Our son had a bit more than flat feet, as his feet were turning in, which was having an impact on his hips etc and could have caused more severe problems when he got older. We were recommended to go to
http://www.therapy4kids.co.uk/ and they've been brilliant.
In general, my understanding is that flat feet aren't a problem in themselves, but if they are a sign of something else, ie a postural/muscoskeletal issue, then it's worth getting your child checked out early, so it can be corrected.
We went for about 3 months, and now just have to go for a check up when he turns 3 to check all is in order