Chametz On Shabbos Erev Pesach
An eruv chatzeros is set up by taking a piece of bread and granting partial ownership of it to all the residents of a courtyard. This bread must be edible. A problem therefore arises on Erev Pesach that falls on Shabbos. Chametz may not be eaten during the second half of the day, and matzah may not be eaten starting at daybreak. What kind of bread, then, can be used for the eruv?
In Place At Outset Of Shabbos
The Gemara (Eruvin 35a) states that an eruv techumin that was in place during bein hashemashos (twilight) is valid for Shabbos even if it was subsequently destroyed. However, if it was destroyed before bein hashemashos, it isn’t valid.
From these rules, it is obvious that an eruv techumin need not be in place for all of Shabbos. It suffices if it was in place when Shabbos begins. Presumably, then, bread used for the eruv need not be edible the entire Shabbos as long as it was edible when Shabbos began.
Indeed, the Meiri (78b), Pri Megadim (Orach Chayim 372, Mishbetzos Zahav, s.k. 1) and Shulchan Aruch HaRav (394:3) all state that the eruv need only be in place during bein hashemashos. Accordingly, Rabbi Shmuel Engel ruled that both chametz and matzah are kosher for use as an eruv if Erev Pesach falls out on Shabbos (Teshuvos Maharash Engel 6:27).
Other poskim disagree. They maintain that the eruv must be set up in such a way that it could remain in place for the entire Shabbos. If Erev Pesach falls out on Shabbos, though, it was obvious from the outset that the eruv would not remain edible for the entire Shabbos. Thus, it would be invalid (Tzofnas Pane’ach, Hilchos Eruvin 6:12).
Children And Matzah On Erev Pesach
The Tchebiner Rav (Dovev Meisharim 1:139, s.k. 2) writes that although using chametz may be debatable, everyone should agree that using matzah is fine since matzah on Erev Pesach is only forbidden to adults and children old enough to understand the story of Yetzias Mitzrayim. Children younger than that may eat it, and, thus, an eruv made of matzah is kosher since it may be eaten during the entire Shabbos.
Although the sugya on Eruvin 35a concerns eruvei techumin, poskim maintain that the underlying principles apply equally to eruvei chatzeros (Beth Yosef, Orach Chayim 394).
Eating the Loaf of the Eruv
Whereas the prevalent custom today is to set up an eruv chatzeros only from time to time, the Maharil would set up a new eruv chatzeros every Erev Shabbos. He would then use the bread for lechem mishneh on Friday night and Shabbos morning.
The Maharam would eat the loaf used for the eruv on Friday night (Hagahos Maimoniyos 8:4; Hagahos Asheri 3:7; Mordechai, Eruvin 490, citing Maharam; Piskei Eruvin 69). Rashi (Berachos 39b), however, seems to imply that it is better to eat the eruv loaf on Shabbos morning (Zevach Tzedek, Chadashos 160).
Poskim explain that it’s preferable to eat it in the morning since people often start Shabbos early. If we became accustomed to eating the eruv loaf at night, those who accept Shabbos early might eat it before bein hashemashos, invalidating the eruv (Shulchan Aruch HaRav, Orach Chayim 393:3; Mishna Berura 394: 4). Based on Kabbalah, the Arizal would eat the bread of the shituf mevo’os on Friday night and the bread of the eruv chatzeros on Shabbos morning (Kaf Hachayim 366:124).
In regard to eruv tavshilin, which allows us to cook on Yom Tov for Shabbos, Shulchan Aruch HaRav (527:25) states that some have the custom to wait until Se’uda Shelishis to eat it (see also Eruvei Chatzeros  by Rabbi Menachem Moskowitz).