As is the case with most superstitions, you are very unlikely to discover an individual who first created the 'step on a crack' one. Since you use the word 'momma', I guess you are American, though the superstition exists in other societies, too. A Google search does, however, suggest an origin in the USA and even the possibility that it had racial overtones. That is, that one's mother would become black rather than break her back! It may be better not to pass that idea on to a 9-year old.