It seems to me that even most adults have an issue with grammar. I fairly often see the same mistakes repeated in essays and normal everyday chat. This isn't just an issue associated with younger children with little or no grasp on grammar, it's a common issue that I see even with graduate students.
I. First, is the "Their, They're, There" mistake.
Their implies ownership.
They left with their jackets on.
They're implies an action.
Today, they're going to the mall.
There implies a place.
Please place that book over there.
II. Second is the "You're and Your" mistake.
You're implies that you are going to do something.
Today you're going to take the dog for a walk after school.
Your implies ownership
You left your pencils on the floor.
III. Third is the "It's and Its" mistake
It's means it is or it has.
It's going to be hot outside today!
Its shows possession.
The cat needs its water bowl filled.
Now the world of grammar should make a little more sense!