well I have never treated a 2 week old calf. I never had the need as I calve in the winter and no flys in middle Alabama in winter. But you may try treating your cows and that should keep the flys off the calves too.
Durosect works well. Try that.
Any of the sprays or pour ons that are permethrins should be fine on claves. The catch is that the cow is going to lick most of it off during the frequent bathing ritual.
We use Permectrin II mixed with diesel in cattle wipes. They're set low enough that the calves regularly get wiped with it. UltraBoss pour-on is listed for use on beef calves.
This is just an observation on my place. I don't have fly problems with calves. Generally don't see flys until sometime after weaning. I can't say why. Possibly the daily baths?