sale deed execution by widow having with a minor

Sir pls suggest me , I had purchased a land from a widow in 1996.(giving her property price)at verbal deal that I will take care of the minor child and I did that ,after 10 yrs of mutation that minor challenging the sale deed mentioning mother was not legal to that disposed property under natural guardianship law.so pls guide me in this regards how can I get justice.The suit land is a joint family property .pls give me in detail.kindly,Thank you.