Pivete is a Brazilian Portuguese slang term that refers to a young kid, often with a slightly mischievous or informal connotation, similar to 'brat' or 'urchin'. It's not a formal word for child, making it more interesting and specific to certain contexts.
In this song, the lyric "arruma um pivete pra chamar de amor" (find a kid to call love) is used provocatively. The singer is telling the ex-partner, who is acting immaturely, to go find someone equally immature to be with, highlighting the contrast between a 'menina' (girl) and a 'mulher' (woman).
This song is the ultimate comeback to an ex who didn't appreciate you. Gusttavo Lima has completely moved on and found someone new, and he's not afraid to spell out exactly why she's better. He tells his ex, "A porta que você fechou, ela chegou metendo o pé" (The door you closed, she arrived kicking it down). What a line! He's saying his new partner entered his life with confidence and passion, right after his ex left him.
The whole song is built on this comparison. He directly contrasts his immature ex with his new, mature partner, saying "'tá aí a diferença de menina pra mulher" (there's the difference between a girl and a woman). While his ex hurt him ("Você bateu, ela cuidou" [You hit, she took care of me]), his new love healed him. He ends with a final, savage piece of advice for his ex: "Enquanto 'cê não cresce, arruma um pivete pra chamar de amor" (While you don't grow up, find a little kid to call your love). It's a blunt and confident track about leaving immaturity behind for a real relationship.