砂場で遊びすぎて寝ちゃった子ども

What does “-chatta” mean in Japanese?

〜ちゃった (-chatta) is the casual, spoken shrink of 〜てしまった. It marks an action as finished and complete, often with a shade of “oops” or “it just happened.” 遊びすぎて寝ちゃった means “(the kid) played too hard and ended up falling asleep.” Plain 寝た just says “slept”; 寝ちゃった adds the sense that it happened all on its own, despite itself.

The present-tense form is 〜ちゃう (食べちゃう, “I’ll just eat it up”); after て-forms that end in で it becomes 〜じゃう / 〜じゃった (飲んじゃった). It ranges from regret (壊しちゃった, “I broke it 😖”) to cheerful finality (全部やっちゃおう, “let’s just get it all done”). Also in the clip: 遊びすぎる, “to play too much,” using 〜すぎる (“too much”).