Muscles can generate the optimal amount of force when they can operate through their intended range of motion. Excessive tightness can also result in injury. Soccer involves the passing and striking of a ball. It also involves a lot of lateral movement and cutting. All of these movements lead to extremely tight inner thighs and outer hamstrings (adductors and biceps femoris).
Try these 2 stretches out and you will see what I mean. They will help you dodge some injury bullets and let your athletes play their best!