童谣 is often translated as “nursery rhyme,” but the rhymes discussed in this KDNet article are really examples of folk doggerel in general. Some of them are pretty dirty:

Arising among the people, rhymes unavoidably had a good deal of sexual content. This was particularly the case in the countryside, where the wanton rutting of cattle, horses, pigs, and dogs greatly contributed to children’s awareness of sex. Children in modern cities do not have this sort of opportunity.

In those days lots of nursery rhymes contained sexual overtones. Children were not overly concerned with all the details; they merely thought them funny, and reciting them was something new and different.