Cross my heart

hey, i am a elementaty teacher in Georgia and for valentines I was going to go thru some heart facts and I
thought it would be fun to talk about the phrase "Cross my heart, Hope to die" and were it came from
so if anyone has an idea- please let me know!! thanks! < Lindsay

From Eric Partridge's Dictionary of Catch Phrases: American and British, from the Sixteenth Century to the Present Day:

"cross my heart!" is a catchphrase of declaration that one is telling the truth: mid C19-20. Originally, a solemn religious guarantee. It is short for "(I) cross my heart and may I die," itself probably elliptical for "(I) cross my heart and may I die, if I so much as tell a lie." [Partridge quotes another writer as saying:] 'Schoolgirls' catchphrase protestation of honesty. "Is this true, Janet?" -- "Cross my heart!"'
Among children in C20, in US as in UK, it is more usual to elaborate "Cross my heart and hope to die!" . . . The shorter form is recorded by the Oxford English Dictionary, 2nd Supplement, 1908; and I have reason to think that it has existed since late C19. . . . cf. the later C20 appeal for veracious certainty, "can you really say, hand on heart, that . . . ?"

From _The Lore and Language of Schoolchildren_ by Iona & Peter Opie:
'Cross my heart', 'Criss cross my heart', 'Christ cross my heart', and smilar vows were usually repeated by girls, and usually with their arms crossed. 'When one crosses one's heart one should do it with arms across the whole of one's chest, touching one's shoulders. Not everyone does it properly', states a 10-year-old Oxford girl.
Little Yorkshire lasses at South Elmsall write: 'When we make a promise to anyone, they say to us "Cross you heart and spit" or "Cross your heart and hope to die". So we wet our finger and make the sign of a cross on our hearts. Sometimes we put our finger on our forehead then on each shoulder, then on our chest. Sometimes they say to us, "Put three crosses on your heart".'
[...]
Cross my heart, / If I ever tell a lie / Put a rope round my neck / Then let me die. / _Aberystwyth_.
[...]
Clasp my hands, / Look at the sky, / Cross my heart / And hope to die. ? _Penrith_.
[...]
Cross my heart and hope to die, / Drop down dead if I tell a lie. [Ruthin]