If your grandfather inherited the property from his father and then his next generation children have again inherited the said property from him after his death and have been enjoying them either joiintly or separately, it may not be considered as ancestral property. in tha tsituation the children of the next generation children can inherit the same out of the share of their respective parents to the extent of their entitlement after the lifetime of their respective parents.
For the self acquired property the grandchildren are not entitled to any share in the property during the lifetime of their respecvite parents who are the legal heirs of their deceased grandfather