both my boys have/had these
matthews (11) has def faded you can only see it now if he is sweaty and hot ie been playing footie etc or upset and crying (not that that happens much now!!) it had pretty much faded by the time he was 4 but gradually got fainter before that.
adam has a much darker, bigger one which has also faded a bit already, but i am not worried about it as i am sure it will go the same way as matthews.
tbh i cant really see it in your pics only the coughting one, so i wouldn't worry about it.