1) The Chasam Sofer writes that if the wife does not keep her Nedarim, then all Nedarim are indeed considered as Beino l'Beinah and the husband may annul them. (However, if she does keep her Nedarim, the husband may not annul them, because such Nedarim do not affect him.)

2) I would also suggest a different answer to your question. If we know that the wife makes Nedarim and does not keep them, then it is preferable that the husband should divorce her. Therefore, even though one could argue that in such a case all Nedarim do become Beino l'Beinah for the reason you said, nevertheless the Torah did not want to give the husband the option of annulling the Neder because that would mean that he might not divorce her. The Torah prefers that he divorce her, because if he merely relies on the fact that he can be Mefer the Nedarim, this is likely to lead to cases where he does not hear about her Nedraim, and this may then cause the deaths of children. Since we do not want this to happen, the Torah did not give the husband the opportunity to annul the Nedarim on the grounds that they are Beino l'Beinah, and this will force him to divorce his wife in order to save his children.

Wishing you a Kesivah v'Chasimah Tovah,

Dovid Bloom

I found that the Tiferes Yakov on Tosfos to Gitin 35b (DH Aval) also writes that if the wife is liable to transgress the Neder, then this becomes Beino l'Beinah because the children might die.

That Tosfos is discussing a vow that the wife made that she will not eat a loaf of bread, but we have good reason to suspect that she will later transgress the vow. The Tiferes Yakov writes that the reason why the husband may annul this vow is that the danger to his children means that it is Beino l'Beinah.

Gmar Chasimah Tovah,

Dovid Bloom

I have found more sources that teach that sometimes Nedarim can become Beino l'Beinah if they might cause the children to die.

1) The Hafla'ah (a teacher of the Chasam Sofer) comments on the Din in Shulchan Aruch EH 80:3 that if a husband made a vow that his wife may not do any work, he must divorce her and grant her the Kesuvah payment.

The Hafla'ah writes that in fact if such a vow was made the husband should be able to annul it. This is because it is a vow which is impossible for her to keep and this means that the wife will inevitably become a wife who does not keep her vows. If so, he should now divorce her as the Mishnah in Kesuvos 72a states. The fact that he should divorce her automatically makes the vow Beino l'Beinah. In other words, any vow which is impossible for her to keep automatically becomes Beino l'Beinah because it will lead to divorce. The Hafla'ah concludes that this matter requires further study.

2) I also found in a Sefer called Shemen l'Asher (in Otzar ha'Chochmah) that the author learns in Tosfos to Gitin 35b (DH Aval) the same way as the Tiferes Yakov that I cited above, that if a wife is liable to transgress the Neder this becomes Beino l'Beinah.

Hence, we now have a Tosfos which seems to say that if the wife does not keep her Nedarim they become Beino l'Beinah, and the Hafla'ah and Chasam Sofer are also inclined to this opinion.

Kol Tuv,

Dovid Bloom

I posed your original question to a Gadol.

He replied that since the husband also makes Nedarim and is not concerned about the possibility that his children may die as a result, it therefore follows that the husband cannot claim that her Nedarim are Beino L'Beinah because she is no worse than him.

[ However if it is known that she does not keep her Nedarim this means that she is worse than him, and this is why the Mishnah states that in such a case he can divorce her without paying her Kesubah]

Dovid Bloom