You could just rule that the beastmaster adds +2 to injury rolls against animals then to represent the higher chance of incapacitating them.
I don't see that being justified fluff wise. Its simple and effective but dosn't make alot of sense.
the beastmaster can charge enemies he does not see if they are not further then dogs's I distance from dog and beastmaster can see the dog.
And you can charge enemies you can't see anyway.
Im sure he means without the I test. We could say anyone within 8" of the honds count as being in line of site for purposes of charging. Call it bloodlust or something other then Sniffing. Its a bit plain though.
I do think something about controlling other animals is the right way to go. Its what a beastmaster is all about.
any enemy animal within 6" counts as having there leadership reduced by 1. Both the beastmaster and its rightful owner trying to control it, its bound to get confused.