I've heard that this phrase: 王様のベッドで寝ちゃおっと is spoken in a child-like way. The full translation is "let's sleep in the king's bed", and the portion that is child-like would be 寝ちゃおっと (let's sleep). This 寝ちゃおっと cannot be translated very easily so could someone explain the nuance of this phrase?
The phrase may be spoken in a child-like way and may have child-like intentions, but I wouldn't call it child-like per se.
The basic form
〜しちゃう (as opposed to
〜する) simply connotes tentativeness or disregard for consequences, similar to how you would say “go ahead and do” instead of just “do”.
This can range from a subtle uncertainty (映画見ちゃう “I'll go ahead and watch a movie”), to mischief (このクッキーとっちゃう “I'll go ahead and take this cookie”), to full-blown recklessness (酔ってるけど運転しちゃう “I'm drunk but I'll go ahead and drive”).
〜しちゃおう removes you one degree from the decision, perhaps revealing your hope of diffusing responsibility for the action. This is similar to how you would say “Let’s go ahead and…” instead of “I'll go ahead and…”, when there is no us and you are the only one going ahead.
Bumping it up to
〜しちゃおっと shifts the nuance from subtle uncertainty to more mischief/recklessness.
(Side note: Although
〜しちゃおう is first-person plural and can be used to prod both me and us, adding a
〜っと at the end restricts the use to a singular context.
〜しちゃおうっと is never used to prod an actual group of people — only me.)
So, if I were to offer a wordier translation just for the purpose of conveying the statement's intent:
Why don't I go ahead and sleep in the king's bed