the correct spelling is "Ben küçükken" . ...-ken of suffix turns the sentence to the clause, and the sentence should continue with a second sentence, which explains the act when you do in the first sentence ... and "ben küçükken" should be translated as " when i was a child." because küçük means small. "Küçük" with a subject "ben" indicates a child in this case. and it should be used such as, "Ben küçükken, burada büyük bir orman vardı." - "when i was a child, there was a big forest here."

ben kucukken. [when i was litte], ben kucutum. [i was little]