My adult children lost their father in 2002 and their paternal grandmother in 2007. Their father was the youngest of three sons, We were divorced for 20 yrs before his death in 2002 he rarely paid child support. My question is, since he passed away before his mother shouldn't her estate still be divided among the three sons and since the youngest sons was deceased his portion should go to his children ? How can I find out how her will was stated and if they have any rights to a portion of her estate I know it has been four years since her death and I don't know if it is to late now but, I would like to know if this is possible.

It will depend on where you live and if there is a will, the one son who died ( or even a living son) can be written out of a will. So no there is not for sure they will get anything.
They can check with probate court to see if a will has been filed. They can check with the families attorney to see if they had one.