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.

Their share is "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. 

Their share is 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 

Fairness of the will of deceased will be decided by the sharia court if there is a complaint. This means children may get from zero to maximum available benefits as discussed earlier.

Since grandchildren will get their shares from their parents, therefore, they are not eligible or otherwise sharia court advise according to the Quran and Sunnah.

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