Salaam Sister
Okay,that is a very good question , you know you should thank them for asking about this because they seem to be very smart kids.

To the point, Now to say if you believe in God then you have to wear the Hijab , this would not be a satisfying answer and God doesn't command us to do things for no reason , He s.w.t said that He has made a reason for everything.
I don't know if my answer will be satisfying as well, and I am saying this because what I will say in the following would probably be typically what I would have said in such a situation also It may be wrong or right .. It's up to you to decide and I am hoping more sisters and brothers would also help you in satisfying these kids answers.
Note before: Never give them the impression that they are kids and they don't matter. Give them a respectable answer that has effort just as you would give it to any adult, so that they can take you seriously and they are more likely to remember this when they grow up, so please keep that in mind.
Into the question: Why do we wear Hijab?
First of all tell them that the Hijab is not obligated except for females who hit the puberty age , it is not Islamic to make a little girl wear Hijab.
The Hijab was designed to protect the women from the evils of men and to protect her from being looked as a merely nothing but a sexual object.
The Hijab was also designed to nourish the character of the women and to make her respect and value her self and not let her become a slave to men and their desires each man who throws in two or three nice words can get a one night stand with a woman who does not value her self and makes her self an easy prey for worthless men.
You can also give example of how women are being used and enslaved today - they are treated like trash , burnt and thrown and have absolutely no value in some societies and even after that some people expect the woman to show off her body to give men pleasure they don't deserve.
It gives the female independence especially that women today face a lot of pressure from the media because they have to look like this or that model in Magazine and if she fails to satisfy the media standards of beauty she wouldn't be getting any respect, but when she wears the Hijab and pronounces her independence then she doesn't have to worry about anything in that matter. People will look at her character and personality rather than valuing her according to her appearance.
This is what I have for now
I hope you receive more and more replies ,If I think of something better Insha'Allah I will be adding more
May Allah s.w.t Help you
Salaam