I believe that Fee and Foo, being Imps, are immortal but with eternal youth. They may have been around for much longer than one would think, Also being immortal would explain Foo taking so much damage yet easily walking it off in "The Negatives of being Positively Charged"

The two probably had many many kids as friends before they met Harvey but they all eventually grew up and forgot about them, and the ones that didn't eventually passed away.

This would explain Fee's jaded personality, and Foo being blissfully ignorant, assuming he forces himself to ignore all of that, and what will eventually happen with them and Harvey.

My prediction for the series finale, is they'll admit this to Harvey who refuses this fate and will do everything he can to prevent it but Fee will point out that countless friends had said the exact same thing in the past.

So Harvey throws a surprise party with every character in the entire show to celebrate everything about Fee and Foo and the theme is "memories"

Harvey tells the two, that there will always be kids somewhere willing to be their friends, and that he himself will encourage his younger sister and his own kids to be friends with them. Whom he'll say he'll encourage to keep a "chain" of them doing the same with their kids and their kids and so on, so Fee and Foo will always have a little bit of Harvey with them. And after the episode is finished, to drive the whole episode home, they'll play the theme intro again at the end. 

Also, one of the "previous kids" was Randl, though Randl chose to forget/ignore his childhood and in turn treated Foo like any bratty kid, or rather he sees Fee and Foo as "Just kids" to spite them for "abandoning him" as he grew up and was forced to accept reality, while they kept on innocently playing games forever. While Foo not as mentally strong as Fee, forced himself to forget all his past friends to ease the pain which may have given him brain damage. (If that's something that can actually happen to an magical immortal being)