The phrase "little monkey" is often used to describe a mischievous or playful person, just like a monkey. It's a light-hearted way of calling someone silly or energetic. Full definition
This cute little monkey shaped sound machine for your little bundle of joy is a unique sound machine which you should not miss. (checkpregnancy.com)
Our 6 and 2 - year old loved being there, our 2 little monkeys jumping on the bed! (luxuryretreats.com)
I tend to refer to my boys as little monkeys more often than not, since they tend to climb over EVERYTHING in our house. (queenofthelandoftwigsnberries.com)