Whether a foul or penalty card is given depends on multiple factors such as the aggression, fitness attributes and the tackling style set. All this should be taken into consideration by you as a Manager.
Note that not all fouls are consequence of dangerous tackles. There are many situations that can result in a foul, such as handball, impeding the progress of an opponent (also called "screening" or "obstructing"), spitting, pulling jerseys, pushing opponents and many, many others. Other examples would be a high kick when an opponent is nearby, or if a player tries to head a low ball that an opponent is trying to kick (then the player who is putting himself in danger would be guilty of dangerous play).