I presume the goal is to allow both the word-by-word rendering and the more-natural-to-English positioning of "mom" first. Report cases where either is missing (as you seem to be fairly new to the course, it'll spare you some angst to just expect a good deal of lingering inconsistency from this course; simply use the report button where applicable and help the course improve; it's very actively being worked on at the moment)
Swahili has 16 noun classes and adjectives will vary depending on the noun. Usiku is in class 11 (the u-noun class) and 'njema' corresponds to class 9/10 (the n-noun classes) - the plural of "usiku" is "siku" (just like the day) and in that case you could use "siku njema" if you find a context for "good nights".
In this context, "usiku mwema" is just the set way to say "good night." That's just how the phrase is. It's not technically wrong to say "uskiku mzuri" but that would not likely be used as a sort of "farewell for the night" context. It's not a perfect analogy, but it might sound like someone in english saying "great night" instead of "good night." It's technically the same but doesn't really sound right..
Such phrases are known as "irreversible binomials" (among other things). Good translation yields a natural result, not simply a word by word rendering, and that includes reversing the order of reversible binomials when appropriate, just as includes reversing the order of noun and modifying adjective when the default order of those differs between languages.