>It becomes child abuse once it is a physical act.
Agreed, so being a physical act seems to be necessary for its prohibition. But we're not talking about child abuse, we're talking about the creation and distribution of child abuse videos, i.e. child pornography. These are two separate crimes.
>Speech is a protected class of expression precisely for this reason: it cannot be abusive.
People can absolutely be abused by speech, even in the common meaning of the word. Spousal and child abuse often happens through speech. Teachers may abuse their students through speech. Cyberbullying is widely regarded as a form of abuse through speech. Threats are themselves speech. If speech is so important as to have no real-world consequences, what makes freedom of speech such an important principle?
> if we define speech as "the communication or expression of thoughts in spoken words"
Freedom of speech (or more accurately, expression) law is much wider than this, and applies to, say, artistic works in which not a single word is spoken, and it also applies to burning flags or silently protesting. There is no need for a word to be spoken, or even a concrete thought - speech can be purely emotive, or a command, or a shopping list, too. Child pornography can easily be considered artistic. The fact that it is art, and therefore expression, is irrelevant to illegalizing it.
>I cannot choose to stab people as a form of free expression, nor torch a building.
The same law that protects freedom of expression also protects freedom of speech. Freedom of speech and freedom of expression are two sides of the same coin. You're noticing that not all physical acts count as speech, and this is certainly true for child abuse, stabbing people, or torching buildings. However, in general, it is not true for videos depicting those things, with the exception of child porn.