Mary, Mary Quite Contrary.
The rhyme is about Queen Mary's persecution of Protestants, she executed over 300 in her short reign. The silver bells and cockleshells refer to instruments of torture, silver bells were thumbscrews and cockleshells - well they crushed an intimate male body part. The Maiden was a crude guillotine later refined to cut cleanly. Her garden grew expotentially as the graveyards expanded to take the bodies of Protestants cut down in their prime.
And now for my interpretation: