Since grandchildren will get their share from their parents, therefore, they are not eligible here according to my own understanding of the related verses of Quran. As your question is vague, therefore, 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) and acceptable 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) and acceptable to their children 

Allah says clearly in Quran

1- Indeed, Allah is ever knowing and wise 2- And Allah is knowing and forbearing

since I am not a scholar of the subject, therefore, I may be wrong