Legal heirship and sale of flat

In the instant case provisions of Indian Succession Act, 1925 (section 31 to 49) applies wherein the 1/3rd share of your wife shall devolve upon you and remaining 2/3rd shall go to children.

However, it would be better to consult a local prudent lawyer for proper analyses of facts/documents, professional guidance and  necessary proceeding.  

In the given facts, provisions of the the Guardians and Wards Act, 1890;  the minor's share could be solved with the Court's permission.

Under Indian law (specifically the Indian Succession Act, 1925 and the Guardians and Wards Act, 1890), you cannot directly sell the flat on your own, even if you paid the loan or cleared the NPA dues using your personal funds.

Even though you are the father and natural guardian, a guardian cannot sell or alienate a minor child's share in an immovable property without prior permission from the District Court (under the Guardians and Wards Act, 1890).

Since you used personal funds/borrowed money to clear the NPA status and save the property from bank foreclosure:

You have a legal claim to seek reimbursement from the estate/property for protecting and preserving the joint asset.

However, this claim must be presented to and approved by the court when seeking permission to sell.

Very well analysed, opined and advised by learned experts, I endorse it.

It is advisable to consult a local prudent lawyer for proper analyses of facts/ docuements and professional advise to proceed as per law.