Here's an example:
The answer is adequate in the specific context by being simple and [superficially] valid. However, it probably does not resist a more academic or biological analysis.
Is there a name for such type of answer?
I found the word "sophistry" but I'm not sure it applies exactly to this.
In an academic context (which the OP explained in a comment), the particular type of fallacy used is called a Red Herring. It is sometimes called the Chewbacca defense, after a South Park episode.
...the speaker deliberately brings up an irrelevant issue.
(Source: Thank you for Arguing, Jay Heinrichs, p. 149)
In a jury trial, a Chewbacca defense is a legal strategy in which a criminal defense lawyer tries to confuse the jury rather than refute the case of the prosecutor. It is an intentional distraction or obfuscation.
As a Chewbacca defense distracts and misleads, it is an example of a red herring. It is also an example of an irrelevant conclusion, a type of informal fallacy in which one making an argument fails to address the issue in question. Often an opposing counsel can legally object to such arguments by declaring them irrelevant, character evidence, or argumentative.
The name Chewbacca defense comes from "Chef Aid", an episode of the American animated series South Park. The episode, which premiered on October 7, 1998, satirizes the O. J. Simpson murder trial, particularly attorney Johnnie Cochran's closing argument for the defense. In the episode, Cochran bases his argument on a false premise about the 1983 film Return of the Jedi. He asks the jury why a Wookiee like Chewbacca would want to live on Endor with the much smaller Ewoks when "it does not make sense". He argues that if Chewbacca living on Endor does not make sense—and if even mentioning Chewbacca in the case does not make sense—then the jury must acquit.
Specious is the word you're after. Another word that fits the bill is simplistic.
characterized by extreme and often misleading simplicity
a simplistic theory of the universe.
[Etymonline]
The mother's answer was disingenuous.
disingenuous - lacking in frankness, candor, or sincerity; falsely or hypocritically ingenuous; insincere
Her excuse was rather disingenuous— https://www.collinsdictionary.com/dictionary/english/disingenuous
The prime candidates are specious and fallacious.
specious = seeming to be right or true, but really wrong or false:
Fallacious = 1 embodying a fallacy a fallacious conclusion a fallacious argument
2 : tending to deceive or mislead : DELUSIVE false and fallacious hopes
From:
fallacy = an idea that a lot of people think is true but is in fact false:
The example you quote is more specious than fallacious, because the false notion about shadow is not (to my knowledge) a widespread belief.
A just-so story is an explanation for some phenomenon that makes some amount of sense at first glance, but has no actual research or data to back it up.
The term comes from a book of fanciful stories by Rudyard Kipling titled Just So Stories for Little Children that made up fanciful and fictional stories about how different animals ended up with their various characteristics. For example, in "How the Elephant got his Trunk," a curious elephant child gets too close to a crocodile, which bites its nose and the elephant child stretches it out into a long trunk while trying to escape.
