Kadesh, Urchatz, etc. (The order of the Seder was instituted by the Rishonim) See Machzor Vitri, Hilchot Pesach, Siman 65 (they instituted to say the details that are practiced during the Seder, such as Kiddush and then washing hands, and they instituted the simanim so that people would know what to do first and what to do later, and to do everything in its place. Therefore, things that are practiced throughout the Seder, such as the four cups, were not mentioned in the simanim (they only mentioned what is to be done now, like Maggid, Rachtzah, etc.), and this is also the reason why hasebah (reclining) is not mentioned, because it is practiced throughout the Seder (we heard from our teacher).
Why did they institute simanim at all—Kadesh, Urchatz, Karpas, etc.? One can say that since there are so many mitzvot on the Seder night—matzah, maror, charoset, karpas, afikoman, and more—and each has its details, such as washing hands before karpas, they instituted simanim so that everything would be remembered and nothing would be forgotten (so it is in Sefer HaPardes, Hilchot Pesach, p. 55, and Machzor Vitri, Pesach, Siman 95).
"And you shall tell your son on that day..." The son-in-law of our teacher (HaGaon Rav Shteinman) was asked by Rabbi Yosef Avraham Wolf zt"l: Why do many people not fulfill the mitzvah of "And you shall tell your son" at all, such as when married sons are guests at their fathers-in-law or eat alone at home and the father has no son at his table? Why are people not careful that every son eats specifically with his father so that the mitzvah is fulfilled? He sent to ask our teacher, who replied: Certainly, the verse "And you shall tell your son" does not mean only literally, that there is a mitzvah to ensure to say to one's son and only to him. Rather, the main mitzvah is to tell the story of the Exodus to others, and the Torah spoke in the present, that one's son is usually with him, but the same applies when there is someone else, as it says explicitly in Pesachim 116a: "If he has no son, his wife asks him," etc. (and from his father) Maran HaGaon Rav Shteinman shlita also heard so (and so it is in Aderet, in a responsum quoted in Sefer HaZikaron Yehuda Lekadsho).
Another time we heard from our teacher shlita the same, with several practical differences, and these are his words: The essence of the mitzvah of "And you shall tell your son" is not to say specifically to one's son, but to say in a manner of telling and relating to another, and the most common is to say to one's son, so the Torah wrote it that way. From the letter of the law, it is enough to tell one's son once in his life, as it says, "And you shall say to your son on that day," etc., and every year on Pesach night it is enough to tell even one's wife or another, as it says in Pesachim 116a, for the law is not to say specifically to one's son, but the main thing is that it should be in the manner of telling. According to this, the practical halacha is that one whose son is not with him does not need to go to his son, for there is no such obligation, and the whole law is that when his son is with him, he should tell him. If one finished the Seder and then his son came to him, it may be a hiddur (enhancement) to say to him "Avadim Hayinu," etc., for after all, it says "And you shall tell your son," but it is not specifically the son, as above.
