Interesting : Is it true that most nursery rhymes have very morbid roots?

Yes, no, and sort of. For instance, this site about nursery rhymes claims that Ring Around the Rosy dates back to the Great Plague of London in 1665. But party-pooper Snopes debunks this as "nonsense," tracing that interpretation to a 1961 book.

The more likely explanation, says one historian, relates to the 19th century Protestant religious ban on dancing. Wikipedia writes that "generally nursery rhymes are innocent doggerel... Urban legends abound with regard to some of the rhymes, though most of these have been discredited." A spoof posted on Snopes parodied these apocryphal theories by claiming Sing a Song of Sixpence was originally used by pirates to attract new members. (Untrue.)

The provenance of many rhymes is interesting enough without resorting to sensational interpretation. Wikipedia calls "plausible," for instance, the theory that Pop Goes the Weasel tells a tale of silk weavers peddling their equipment to pawnbrokers for drinking money. This NPR story explains that in the language of Henry VIII's time, Goosey Goosey Gander associates the Catholic Church with prostitution. As for "Sing a Song of Sixpence," it tells the story of Henry VIII's ill-fated marriage to Anne Boleyn. And a complicated set of references in Yankee Doodle really just boils down to trash-talking between the British and Americans during the Revolutionary War.

There is at least one overtly morbid book of nursery rhymes: Heinrich Hoffmann's Der Struwwelpeter. These tales of horrific punishment for children who exhibit undesirable behavior must have traumatized many an unsuspecting, nose-picking tot.