Translation:We wanted to make the boy happy.
Yeah, in this example it's singular, so "Ich kenne den Jungen" means "I know the boy." On the other hand, "I know the boys" would be "Ich kenne die Jungen"; in an example like this, the only way to tell whether "Jungen" means "boy" or "boys" is by looking at the preceding article. ("Den" is the accusative masculine definite article, while "die" is the accusative plural definite article.)
Similarly, in the example here, "Wir wollten dem Jungen eine Freude machen" means "We wanted to make the boy happy" (note the singular "boy"). If you wanted to pluralize "boy", changing the meaning to "We wanted to make the boys happy", you would change "dem" (the dative masculine definite article) to "den" (the dative plural definite article): "Wir wollten den Jungen eine Freude machen."
So yeah, a fair amount of grammar knowledge is required to translate this sentence correctly, in addition to familiarity with the "eine Freude machen" idiom.
Yes, this common German phrase does not directly translate well to English. It should simply be committed to memory:
(jemandem) eine Freude machen
[literally, “to make a delight (for someone)”]
to make (someone) happy
There are other ways that one can say “to make (someone) happy” in German, including the literal translation “(jemanden) glücklich machen”, but this is a useful one to remember.
It is also possible to use the “Freude machen” construction without the “eine”, but I believe it has a subtly different meaning. My unabridged dictionary translates “(jemandem) eine Freude machen” as “to make (somebody) happy” and “(jemandem) Freude machen” as “to give (somebody) pleasure”. I believe the latter is not commonly used with people (so you couldn’t really drop the “eine” in this sentence), but rather with things and activities and so on—for example, “Es macht mir Freude, anderen Menschen zu helfen” (“It gives me pleasure to help other people”).