child's mind; childlike innocence; naivete
Refers to the pure, innocent, and straightforward way of thinking characteristic of a child. Often used in expressions like 童心に返る (return to one's childlike innocence) or 童心を忘れない (not forget one's childlike heart).
Even as an adult, I don't want to lose my childlike innocence.
He returned to his childlike mind and played innocently.
When they come to the amusement park, even adults return to their childlike innocence.
He is someone who never forgets his childlike heart, even as he ages.
That toy appeals to the inner child of adults.