bad boy; naughty child; brat
Refers to a mischievous or ill-behaved boy, often with a slightly old-fashioned or literary tone. Can be used affectionately or critically depending on context.
The neighborhood brats climbed over the fence again.
Apparently he was an uncontrollable bad boy when he was a child.
悪童 is a slightly literary, gentler word for a mischievous child, whereas ガキ is blunt, casual slang.
Compound of 悪 (aku, 'bad, evil') and 童 (dō, 'child, juvenile'). The reading is on'yomi, and the word has been used in literary Japanese to describe a mischievous or delinquent boy.