How can I examine the significance of teen or child characters portraying the major thematic and social and political concerns of Romeo and Juliet...

The first issue we need to think of here is that our ideas of age have changed over the centuries. In the Middle Ages and Renaissance, Juliet, at the age of 13, would be considered of normal marriageable age, while during the Victorian era aristocratic girls traditionally "came out" by being presented to the Queen at ages normally ranging from 15 to 18. Now, of course, Juliet would be considered far below a legal age...

The first issue we need to think of here is that our ideas of age have changed over the centuries. In the Middle Ages and Renaissance, Juliet, at the age of 13, would be considered of normal marriageable age, while during the Victorian era aristocratic girls traditionally "came out" by being presented to the Queen at ages normally ranging from 15 to 18. Now, of course, Juliet would be considered far below a legal age for marriage. 

Literary renderings of young female characters in both periods typically represent a combination of innocence and passivity, demonstrating the gender inequality of the period. Often they are portrayed as victims, and their travails serve to evoke pity from the audience. Nancy, despite being a thief, combines the notion of the young female as a positive moral compass with the girl-in-peril trope. 

Many of the younger women serve to illustrate social inequality and injustice in their roles as victims. One could argue that much of the tragedy in Shakespeare's play could have been averted had Juliet had the freedom to choose her own husband.