Some have the minhag to refrain from eating matzah starting from Rosh Chodesh Nissan. However, strictly speaking the prohibition only starts from erev Pesach. The Mishnah Berurah (471:12) writes that even on erev Pesach the prohibition only starts from amud hashachar. Therefore, those who are lenient to eat matzah until erev Pesach, may eat matzah on Friday night. [R’ Chaim Brisker (Koveitz Hosafos to Haggadah Shel Pesach MiBeis Levi pg. 30) proves from the Mishnah in Pesachim (116a) that it is permissible to eat matzah on the night of the 14th. In the Mah Nishtanah we ask: “On all other nights we eat chometz and matzah and on this night, we eat just matzah”, we see that on all other nights we may eat matzah, and there is no halachah that on the night of the 14th of Nissan we can’t.] However, the Igros Moshe (Orach Chaim 1:155) writes, that since the Magen Avraham (471:6) holds that one should refrain from eating matzah on the night of the 14th of Nissan, practically one should be stringent. So writes the Ben Ish Chai (Year 1, Parshas Tzav 26 and Shu”t Rav Peolim, Vol. 3, Orach Chaim 27).
Regardless, young children who don’t understand the mitzvah of Sippur Yetzias Mitzrayim may lechatchilah eat matzah on erev Pesach (Rema 471:2). However, once they are of age that they understand the mitzvah of Sippur Yetzias Mitzrayim then it is prohibited for them to eat matzah. However, certainly one can be lenient with them on the night of the 14th.
