Aggressive soccer play in young girls is rare. I think it's more rare than I see in boys of the same age. I'm unqualified to make that statement as fact, it's just what I tend to observe in the years I've been coaching.
For those rare girls that possess those traits, the field is their playground. They become the standout stars, the girls who seem to have what it takes to win the ball, win the game. I see so many girls with great ball handling skills, girls who have a great head for soccer and yet when they get on the field for an actual game, they tend to get lost in the crowd of girls on the field. They'll have moments of brilliance in the open field yet once they come into contact with another player, it all goes south.
When I first started coaching, I felt like you were either born with an aggressive nature on the field or you weren't. I felt it was part of who you were in life. However, over the years I've come to realize that you can show them skills and techniques to move in on the ball, to squeeze out competitors in a safe, legal way and over time, it's just another skill, another tool in their bag, and has nothing to do with aggression. The two, aggression and technique, aren't neccessarily inextricably linked.
Two Cones, Two Players, One Ball
Here's a simple drill that you can use to teach moving a player off the ball. Two lines of girls 10 feet apart facing the same way. 20 yards away you will have two cones also 10 feet apart. One line of girls has the ball, the other doesn't. Upon whistle, the girl with the ball dribbles diagonally to the cone while the other girl's goal is to win the ball and continue dribbling to the other cone. The two girl's lines intersect in the middle so it's an opportunity for the girl without the ball to move the girl with the ball off and get the ball while the girl with the ball must try to protect.
These girls must have clear instructions to not stop with the ball or kick it too far ahead. They must dribble the ball (showing control) past their respective cone in order to score a point. Having them keep their own score is important too because the girls are competing against each other for the ball with a clear goal in mind, TO WIN. Once they are done they get back into the queue on the opposite side they last played so they get both sides of the coin.
During this drill you can stop the play, show the girls how to win the ball and how to protect it. Using the shoulder to shoulder technique to move a player off the ball.
If you do this drill for 10-15 minutes each practice, I guarantee you will see these skills cropping up during games.
The value of course is two fold, first, it's giving them the tools to feel confident in their challenge so they are more likely to do it. It also is a chance to talk to them about how even if they didn't succeed in taking the ball, they have disrupted the player by slowing them down or otherwise moving them off their line and thus allowing another teammate to move into the play and help.
I discuss implementing this drill in more detail here.