The father is the primary guardian for a legitimate boy and unmarried girl and their property, while the mother is the secondary guardian. However, the mother is the primary guardian for all children under the age of five. Natural guardians can take actions that will benefit and protect the minor and his or her property. The welfare of the minor will be the primary consideration in the appointment of a guardian.
In your case both are almost under 5 years of age hence seeking custody at this age of the children may not favor you though your true intentions are to keep the children back with you for providing them with good welfare activities including good education and better living facilities.
If you have decided to file a child custody case along with RCR, you can apply the provision of either GWA or HM Guardianship act in anapplictionalongwide the RCR case or a separate case where the children currently reside. In any case you can always file an Interlocutory application seeking visitation rights to visit your children periodically as an interim relief.