Umm... what does "Feliz Cumple" mean? "Feliz" is "Happy", right? "Cumple" sounds a lot like the German "Kumpel", meaning "buddy". "Happy buddies"?
Ah, that could mean "Have a nice party with your folks".
Or perhaps it derives from "crumble" - "Happy crumbling!" or simply "Enjoy your birthday cake and all those cookies!"
Yeah, sure, it has to be one of those. What else could it mean?
Cheers!
Markus