Is it mały (not mała) as chłopiec is masculine? What is the neutral for mały/mała?
Would you actually say this in practice - would you use chlopczyk?
"mały chłopiec" is fine. "mały chłopczyk" as well. "little" is subjective after all, you could use it for a 2-years-old, you could use it for a 6-years-old.
With "little boy" as the English sentence, we even accept just "chłopczyk" on its own.