Translation:El niño y la niña.
is there any difference between chico/chica and niño/niña?
Not really, but niño and niña are more formal words.
Chico/niño is boy
Chics/niña is girl
... sittin' in a tree...
What is a pibe?
It's slang for child, equivalent to "kid" in English.
And as i know it's only used in Argentina