I am thinking of going into pediatrics to treat infants, children, and adolescents.
That's awesome!  General pediatrics or specialty?
Pediatricians could do either, but I think I would like to be a primary care physician rather than specialize in something like neonatology.
What kind of schooling would that require?
Lots.  I would need to graduate from medical school, then have postgraduate training in pediatrics.
Do you know when the first pediatriac practice began?
No, but had to be a long time ago, since the word pediatrics is derived from two Greek words: pais for child and iatros for doctor.