Yekke,
If they are Israelis it is not surprising that they speak Hebrew. Even those who speak Yiddish at home speak Hebrew as very few people outside Chassidic communities have any knowledge of Yiddish (Litvish rabbanim use Hebrew although sometimes they use the Litvish pronunciation). I personally was in a shiur given by an Israeli who learned in Kol Torah (Rav Shlomo Zalman’s yeshiva) and he odes not know Yiddish at all. Kol Torah was, in fact, the first Chareidi yeshiva (Rav Kook started his yeshiva beforehand) to go over to Hebrew (eighty years ago) and the Chazon Ish said that the language of instruction should be Hebrew because so few understood Yiddish (not to mention Sepharadim and bnei Eidot HaMizrach).
BTW, the Raavad is quoted by the Sheeta Mekubetzet as saying that it is doubtful if someone who does not learn in Hebrew fulfills the mitzva. One of the reasons why the Bnei Yisrael merited leaving Egypt was that they spoke Hebrew and not “Judeo-Egyptian”.
Mammele,
Even if they did know Yiddish they probably would have preferred English as they would have trouble expressing themselves in Yiddish. This is a common phenomena among children of olim.