Hi
1) It is a noble gesture from your side that you are taking care of the children in all aspects.
2) Under your personal laws , Guardianship in marriage (jabar) the following persons can act as guardians in the marriage of a minor, in the order of enumeration:-
(i) Father
(ii)Father’s father
(iii) Full brother and other male relations on the father’s side
(iv)Mother
(v) Maternal relations within prohibited degrees
(vi) Qazi or the court
3) Under Muslim personal laws applicable to both Sunni's and Shia's in the absence of the father, the grand father is a testamentary guardian unless and until the guardianship is granted to you by the Court.
4) In order to have your name as legal guardian in the passport of the children, you should make an application to the courts under Guardians and Wards Act, 1890, to the District Court.
5) In general and more so in your case, the District Court may appoint you as a guardian of a minor child’s person on the grounds of necessary for the welfare of the minor, taking into consideration the age, sex, wishes of the child as well as the wishes of the parents and the personal law of the minor.
Hope this information is useful.