Is it a halachic requirement for children do act in the minhag and derech of their parents. Can a boy decide to take minhagim and act chasidish if his parents are from Lithuania and follow litvish minhagim and nusach?
A person should not change from the custom of his ancestors, the more so with regard to the nusach of prayer. However, some (Chassidic) poskim write that one may change from Ashkenaz to Sephard, and some (Lithuanian) poskim write that one may change from Sephard to Ashkenaz. If a parent is makpid that a son should not change his nusach, the son should heed his father’s kepeidah.
Sources: Magen Avraham (68) writes that a person should not change his nusach. Avodas Hakodesh (Chida, Kesher Godal 12:9) also writes that one should not change his father’s customs, for each nusach of prayer has a source (a “gate” in heaven); concerning customs in general, see Shaul U’meishiv, vol. 3, 1:247, who writes that one should not change customs.
Concerning nusach ashkenaz and nusach sephard, the Chasam Sofer (no. 15) famously writes that all nuschaos are equal , whereas Divrei Chaim (2:8, also quoting from Mishnas Chassidim) writes that the nusach of Arizal has a special virtue, though he only writes that somebody unclear about his own nusach should adopt it. In spite of the above mentioned Magen Avraham, Maharam Shik (CM 24) writes that somebody whose soul desires nusach sephard may change over; on the other hand, Iggros Moshe writes that somebody may change back to the original nusach ashkenaz, but not vice versa. See also Minchas Elazar (1:11); Keren David (19); Maharam of Brisk (2:28); Zichron Yehudah (15).