"This snack is not that delicious."
おかし can refer to sweets in general, including baked goods. (Actually, I think "snack" is kind of an odd translation for it.) あめ is more like hard candy.
おかし can also refer to savory snacks like potato chips.
[Edit: Google Image Search of お菓子 brings up jagariko and potato chips, not sure why someone disagrees that okashi is not for savory snacks if you could please explain? I especially find when children get an "okashi" prize, it's usually umaibou, another savory snack.
QUESTION: If あまり means "not really," and おいしくない is "not delicious," how is the sentence not a double negative?
Wouldnt this be, " This snack is not really not delicious." Meaning it's not so bad tasting?
あまり is used with negative verbs/adjectives to mean "not really". It's the combination of あまり and おいしくない that makes it "not really delicious".
Good to know, thanks! Jisho also has a page for お菓子 and it's listed as JLPT N5, whereas 菓子 is listed as JLPT N3, so that seems to agree with what you were saying. https://jisho.org/search/%E3%81%8A%E8%8F%93%E5%AD%90