As a trainer I am constantly asked this specific question "How come, or how can I make my son / daughter more aggressive".
The thing that most parents aren't realizing is that it will come with age, and confidence. Many of these kids are still under the age of 13 and have still yet gone through puberty. As a kid gets older, and continues to develop his/her skills they gain more confidence in their selves. When you are more confident you do things with less hesitation and not fear making a mistake, because you put in the practice. The result of that is having someone who is generally more aggressive because he/she believes in themselves and what they are capable of.
On the other hand, when you have someone who has not put in the practice time and repetitions and a situation occurs that they are not ready for, they will be less aggressive because they don't want to make that mistake.
This question at the end of the day will always be something that every parent continues to ask. It is generally more appealing to a coach, when a kid is not scared and more aggressive. We all want that kid who will dive for any loose ball, attack the basket with no fear, or shoot the ball no matter the situation. What parents have to realize is that everything takes time. It is more beneficial for a kids development to work on their skills, then to focus on him/her begin more aggressive. As kids play more, and grow it will naturally start to come. To pressure your kid now and harp on them to be more aggressive will most likely just cause them to dislike the sport and possibly quit. Allow them to have those growing pains, and be there to guide them. They eventually will have no choice but to be more aggressive as competition starts to get better.
Steps to help your child:
Getting them to practice their skills, so they can be comfortable (confident) when they are on that "Big Stage"
Let them make mistakes, it is the best teacher!
Remember they are still young and it will come with time
Teach them the proper way to play the game instead of teaching them to be more aggressive
Just remember, everyone is not built the same and each kid brings a different skill to the table. Some are better passers, better leaders, others are more aggressive, and some can shoot better. At the end of the day that is what formulate's a team! The more rounded your kid can be the better. So don't focus on just one aspect of the GAME!