They are holding pictures of their children because their children .

Not pretty sure of what you´re trying to achieve here.

Explanation: I will assume that this is an incomplete question or sentence, and therefore, we´re going to take this as a grammar issue.

Let´s stay as pro as we can.

First of all, if you´re writing a text/sentence without any intention of being "formal", then apostrophes are allowed. BUT, if you´re writing something of formal nature then you have to do it all the way long (they are instead of they're) Sounds picky, right? Most of us ignore this kind of details, and with all reason.

And the comas! Please don´t forget the comas:

THERE ARE HOLDING PICTURES OF THEIR CHILDREN, BECAUSE THEIR CHILDREN

They are holding pictures of their children because they're children.

Explanation:

That grammar is terrible. There is a difference between: there, their, and they're.

the percentage of students at the school need special diets is

c) 10 percent