Your analogy seems good!
In terms of overall approach to this (and other) Necessary Assumption ....
1. Read it / find the argument core
2. Did you hear a term shift / missing link / missing idea?
(if so, that's a good prephrase, and you might want to scan answers to see if that's what the correct answer is rewarding)
If not, (or if you want to be super thorough)
3. Treat it like Flaw or Weaken and try to come up with some objection to the conclusion.
About 30 or 40% of correct answers on NA are just ruling out an objection (Defender answers).
We usually don't like these answers much on a first pass unless we've already considered the objection that's being ruled out.
Since the author's conclusion is
"Speech acquisition is entirely motor control"
our objection should help us argue that
"Speech acquisition is NOT entirely motor control"
i.e. "something BESIDES motor control is involved in the speech acquisition process"
The author is arguing,
GIVEN THAT young kids don't have the motor ability to produce the sounds of speech,
WE CAN CONCLUDE THAT speech acquisition is entirely a motor control process
We are thinking through the Opposing Counsel lens,
The author is arguing,
GIVEN THAT young kids don't have the motor ability to produce the sounds of speech,
HOW CAN WE ARGUE THAT speech acquisition is not entirely a motor control process?
It's pretty easy to make that argument.
Just say that speech acquisition also relies on our brain development. An infant's brain is still developing, just as its motor abilities are. Maybe speech acquisition necessitates motor control as well as a fully formed prefrontal cortex.
After all, speech acquisition isn't just about producing sounds that match words --- it's also about understanding conceptually that words map to certain objects / actions / feelings.
People with aphasia can think the ideas they want to say, but they've lost the motor control to speak those ideas effectively.
It seems like speech acquisition has mental prerequisites as well as mechanical ones (a functional tongue / vocal tract / etc.) .
Some five year olds might have developed the motor control to make all the sounds of a language, but don't have the mental functioning to acquire speech.
So our potential objection comes from the realm of, "Couldn't there also be mental requirements that a person has to meet?"
(A) rules out that objection. That's why it's correct to say the author is assuming that.
When we negate it, it weakens the argument.
If speech acquisition is NOT just about ability to produce sounds, then the conclusion sounds wrong. It sounds like it's not ENTIRELY about motor ability.
Hope this helps.