My in law invited us over for dinner. But she only cooked curry and white rice! My 5 years old boy don't take spicy food so I fed him before visiting my in law. However , my in law were not happy , saying that my boy don't appreciate her cooking (curry)! I was dumbfounded !

If only you had known earlier, could have brought some other side dishes over as "contribution". I'm guessing she's just disappointed (perhaps she spent a lot of effort cooking the curry) that your boy did not get to sample her cooking. If I were you, I would probably just tell her that your boy doesn't eat curry when you cook it too. So, it's nothing personal and hopes she understand. ;)