The best explanation I've heard is Rich Hickey's in the Value of Values...i.e., what we usually want when we do something is the next value and changing something and then reading it is more complex than just getting it. I'm not necessarily saying sit your child down with the video, but watching parts of it with your child might be worth it...or watching it and explaining it.
As for a first language, my opinion is Racket unequivocally. The Beginning Student Language is entirely functional and the ecosystem offers a gradient from there to just about any point on the spectrum...from Typed Racket, Datalog, Lazy Racket and Algol 60...all within the same package, DrRacket.
Racket is also designed as a teaching language and extends that part of Scheme's mission. It has decades of educational research in its history.
As for a first language, my opinion is Racket unequivocally. The Beginning Student Language is entirely functional and the ecosystem offers a gradient from there to just about any point on the spectrum...from Typed Racket, Datalog, Lazy Racket and Algol 60...all within the same package, DrRacket.
Racket is also designed as a teaching language and extends that part of Scheme's mission. It has decades of educational research in its history.
Good luck.