The interpretations are correct but there is still a bit more to get from this passage:
is correctly translated as:
"As to how my mother felt at the time"
but the "、" can be seen as functioning like "は"：This phrase is the topic of the rest of the sentence. It links directly to "ことだった。", as in AはBことだった (ie A、Bことだった。)
As virmaior says:
知るべく＝something I could know [but in this case understand]
It is worth mentioning that べく is usually associated with "must" but in this case it is closer to "should", which is in turn closer to "could". There best way I can think of to make this point clearer is to compare "There is no way I could have known..." vs "There is no way I should have known". They are not exactly the same but often you can say either without changing the overall meaning.
The next grammatical point is the expression 〜べくもない. I think there is another JLPT N1 point here but the best I could find quickly in my old textbooks was several expressions that include ~もない such as ~出来そうにもない, where the "もない” makes the negative sense more absolute (-> "There is no way I could know" rather than the slightly weaker "I could not know").
And finally the sentence ends in past tense, "だった". The writer could have used "だ" and the meaning would not have changed but when relating past events the plain form is to describe "the stage" or circumstantial matters. This is not a circumstantial matter. It would not surprise me if the complete text you are looking at describes other more circumstantial matters in the plain form.
We end up with a more literal translation such as:
"As for how my mother felt (then), it was not something that the child I was at the time could have known."
Which equates to the more natural:
There was no way that I, as a child, could have known what my mother was feeling back then.