me personally wouldnt reach for the antibiotics initially and wouldnt isolate the young birds showing signs, starve your entire team for a minimum of 24 hours (36 if needed) then gradually feed them up on a depurative, start of with a quarter feed then half unitil you get up to a full feed, then slowly switch them over to a light mix (gerry plus) and you will find most will pull through if you nab them in the early stages.
every morning if you have say 36 young birds then pick up 6 at random and feel there crops, all being well should be empty.
but i do agree that you will have to be aware of the secondry infection, if using any products then belgica de weerdt would be my first choice.
if you still have a youngster that hasnt responded to this then i would mix up some baytril in a small tupperware dish and syringe 2-2.5ml down the throat twice a day for 7 days all the while the youngster still mixing with the team.
i did this with a young bird last year, he flew young bird programme out to 204 miles and last week was my time in from eastbourne as a yearling 419 miles, he finished 1st Club,7th Fed, 27th Section,82nd Open SNFC... 3841 birds