As per your statement your grandfather has divided his property into two shares by allotting one such share to each of his son under the Stamped Partition agreement. you have not mentioned whether said Stamped document is Registered or not. As per Sec. 17 of Indian Registration Act any document evidencing transfer of immovable property value more then Rs.100/- requires compulsory Registration, In your case you have not stated that the stamped document evidencing Partition is registered or not. However if it is not Registered you have to get the said document Re-validated through concerned District Registrar.
Further if your fathers name is mutated and reflecting in the Revenue Records in pursuance of the said Partition showing your father name as owner / pattadar and so also possessor since last 25 years , your grand father cannot re-claim the property and your father acquires rights by way of partition and so also your father cannot claim the rights by way of adverse possession in view of your father being in physical possession over the allotted property for more then shall 12 years .
so far as the Grand children claiming rights in the property held by grand father, if the son of the Grand father dies leaving his wife & children ( i.e. wife and Grand sons/ grand daughter) then such children and wife shall be entitled to claim the rights of deceased son. However if the son of grand father is alive his children and wife will not get any share.