As I am not a scholar of the subject, therefore, I may be wrong but here is the answer according to my own understanding of the related verses of Quran. 
As your question is vague, therefore, the following are the possible cases.  

Case #1: If the children are Boy(s) and Girl(s) (2/3 + 1/3 = 1) - Two gender 

Case #2: If the children are Boy(s) only = They will get everything if there is no will left by the deceased.

They will get the minimum of "for the male, what is equal to the share of two females = 1/2 + 1/2 = 1 (again)" of the estate if the will left by the deceased is not reasonable (Quran 2:180) or fair to their children.


Case #3: If the children are Girl(s) only = They will get everything if there is no will left by the deceased. 

They will get minimum of 1/2 or 2/3 of the estate (depends on # of girls) if the will left by the deceased is not reasonable (Quran 2:180) or fair to their children 

Since grandchildren will get their shares from their parents, therefore, they are not eligible.

Allah says clearly in Quran

1- Indeed, Allah is ever knowing and wise 2- And Allah is knowing and forbearing 3. You know not which of them are nearest to you in benefit