My sons were guests in Bnei Brak. The hosts forgot to dohafrashat challa before Shabbat. On Shabbat, the ba’al habayitseparated some challa to be burnt after Shabbat. Was it okay for my sons to have eaten? Answer:
It is not clear what you mean by "separated some challa," and the situation for your sons depends on that.
Among the mishna’s (Beitza 36b) long list of Rabbinic prohibitions of Shabbat and Yom Tov is taking terumot and ma’asrot, which includes the taking of challa (which is likewise theoretically slated to go to akohen). Therefore, if your sons’ hosts did hafrashat challa on Shabbat, they apparently acted improperly. We do find leniency for takingma’asrot in a case where one does not have alternative food to eat for Shabbat, due to the mitzva of eating on Shabbat (Shulchan Aruch, Orach Chayim 261:1; see Mishna Berura ad loc. 4). However, that is only to do so during bein hashemashot (twilight, at the time when it is adoubt whether it is day or night). (One can contemplate some leniency as to when bein hashemashot ends in regards to this question, considering the issue is a Rabbinic prohibition, but we will not analyze all the opinions as to specifics.) On Yom Tov, it is permitted to takechalla if the obligation began (with kneading) on Yom Tov (which is prohibited on Shabbat) or by making more dough and taking off from it on the existing dough/challa (Shulchan Aruch, OC 496:3).
However, regarding your sons, even if their host did hafrashaimproperly, they were still allowed to eat the challot. This is because if one took ma’asrot improperly on Shabbat unintentionally (including out of ignorance of the halacha), the food may be eaten (Gittin 54a). (It is a fascinating question why we do not say that since when Shabbat started the food was not fit to be eaten, it should be muktzeh. However, it apparently is not muktzeh – see Tosafot, Shabbat 43a, Shut R. Akiva Eiger II:103; Minchat Shlomo 62.11).
Perhaps the hosts did not actually take challa but left enough of the challa (loaf) over to take challa from it after Shabbat. (There is a discussion among the Rishonim whether one may eat everything except the part that will become challa or whether he must leave over enough to take challa off and still have some bread that is permitted to eat – see Tosafot, Beitza 9a.) This practice has a strong basis, but if this is what they did, they misapplied it. Shmuel says (Beitza 9a): "Regarding the taking of challa of chutz la’aretz, one may eat now and take off thechalla later." This is different from the situation regarding ma’asrot, where until the ma’aser has been taken, the produce is forbidden astevel.
Shmuel clearly states that this halacha is true specifically regarding chutz la’aretz, not Bnei Brak. However, one might want to suggest that it might apply even in Israel of our time, for the following reason. The Tur (Yoreh Deah 323) explains that Rabbinic-level challahas several leniencies, including that it does not have a status of tevel, as above. Challa outside Eretz Yisrael is certainly only Rabbinic (Rambam, Bikkurim 5:8). However, there is significant Talmudicmachloket as to whether challa is of Torah or Rabbinic level in Eretz Yisrael (or parts thereof – see Rambam ibid.) of our times. The more accepted opinion is that even in Eretz Yisrael, challa is now Rabbinic (see Beit Yosef, YD 322). So can we consider being lenient to eat the bread from which challa will be taken later in Israel of our times? The answer is that we cannot be lenient. The Beit Yosef (YD 323, and in the Shulchan Aruch, ad loc.) rejects the above reading of the Tur and proves that, even if both are Rabbinic, challa in Eretz Yisrael is of a higher level and is modeled after the Torah law which did (and will) exist there. This is in contrast to challa in chutz la’aretz, which has "no root in Torah law." Therefore, if challa was taken only on Motzaei Shabbat, then that which your sons ate had a Rabbinic status of tevel, which of course is forbidden to eat.