Right of Grand son/daughter

when the child is brought as a muslim the child till thier majority will be muslims under the sharith act of 1935. after the children at the age of 18years if they wish to stay in muslim religion, the muslim succession rules will apply if not interest to stay as muslims means  the Indian succession act 1925 will apply and property will be disposed according to indian succession act because the parents of the children are married under the special marriage act

i want to know what is the inheritance of the muslim grand parents