What is the criterion for the obligatory kindness towards parents? And must I remain smiling in their presence all the time?

The meaning inquired about is clarified in Allah’s saying: "And lower to them the wing of humility out of mercy." Al-Tabari interpreted this as obeying them in what does not involve disobedience to Allah, and not opposing them in what they love. It is obligatory to obey them in matters that are beneficial to them and do not harm the child, and to avoid everything that hurts them, whether in word or deed, which would constitute disobedience. As for cheerfulness, smiling, and bringing joy to them, these fall under the general meaning of righteousness, which includes showing kindness to them, striving for their benefit, and refraining from harming them. Its comprehensive meaning is to deliver all possible good and ward off all possible evil, according to one's ability.