But, if we cut to what I believe is the heart of your point, then yes, she absolutely should. Let's scale the problem up for a moment. Say instead of 5 it's 500. Or 5 million. Or the entire rest of humanity aside from the mother and her baby. At what point does sacrificing her child become the right decision? Really, this boils down to the idea of shut up and multiply.

Never, in my opinion. Put every other human being on the tracks (excluding other close family members to keep this from being a Sophie's choice "would you rather..." game). The mother should still act to protect her child. I'm not joking.

You can post-facto rationalize this by valuing the kind of societies where mothers are ready to sacrifice their kids, and indeed encouraged to save another life, vs. the world where mothers simply always protect their kids no matter what.

But I don't think this is necessary -- you don't need to validate it on utilitarian grounds. Rather it is perfectly okay for one person to value some lives more than others. We shouldn't want to change this, IMHO. And I think the OP's question about donating 100% to charity, at the detriment of themselves, is symptomatic of the problems that arise from utilitarian thinking. After all if OP was not having internal conflict between internal morals and supposedly rational utilitarian thinking, he wouldn't have asked the question...