If all these excuses are not enough, then I want to dedicate this book to the child whom this grown-up once was.
If we look at the corresponding gap in the relative clause:
- I want to dedicate this book to the child(i) [ whom(i) this grown-up once was __(i) ].
we'll see that the gap "__(i)" is linked to the relative pronoun "whom", which is then linked to the antecedent "child".
Note that the relative clause (which is within the brackets and is in italics) is modifying the noun "child".
And so, the relative clause could have the interpretation of:
- this grown-up once was the child
The reason why the relative pronoun can be in accusative case (e.g. "whom") is because the gap itself could correspond to an accusative pronoun, such as "her". For example:
- this grown-up once was her
And so, the use of the relative pronoun "whom" is possible here in the original example sentence.
ASIDE: Notice that in the original example, the relative pronoun "whom" is not the subject of the relative clause (the subject of the relative clause is "this grown-up"). And so, usually, that word "whom" could possibly be "who" or "that" or nothing at all--it is up to the writer as to which word or nothing that they wish to use as the relative word/pronoun in the original sentence.