The first question, that came to my mind is: are the piccolo players you know skilled in circular breathing? If not, the beginning should be revised, no piccolo player can play these high a's without breathing and hence breaking the obvious attempt to maintain the pitch uninterrupted.
I personally believe this is more appropriate to be the movement of a Suite, rather than a Symphony.
The piece needs some modulations to other tonalities. The varied use of orchestration and dynamics help this music a lot, but there is no real climax or waypoint towards a certain progression of the musical material.
You are relatively new in the field of composition. I remember my beginnings, and this is more ambitious than mine were though.