cross one's heart

[cross one's heart] or [cross one's heart and hope to die] {v.phr.}, {informal} To say that what you have said is surely true; promise seriously that it is true. - Often used by children in the longer form. Children often make a sign of a cross over the heart as they say it, for emphasis.

"Cross my heart, I didn't hide your bicycle," Harry told Tom.

"I didn't tell the teacher what you said. Cross my heart and hope to die," Mary said to Lucy.