I know "cat's cradle" as a string configuration, where various designs are created by interweaving a longish loop of string around the fingers of both hands. This is a childhood pastime.

"Silver spoon" may have been chosed to rhyme with the next line more than for any other reason. There is a saying "born with a silver spoon in one's mouth", meaning born into significant wealth, and there is a nursery rhyme with the line "and the fork ran away with the spoon", but I don't see how either of these really relates to the song.

"Little Boy Blue" is a nursery rhyme.

"The Man in the Moon" is a characterization of the moon as a face, which it sometimes resembles when full. It's more of a simple childhood concept, dispelled with age and knowledge.

My best guess is that these are a somewhat cryptic and poetic reference to childhood. The meaning might be better conveyed with German cultural images of childhood, but unless "cat's cradle" is known by an equivalent name in Germany, that would mean changing the song's title.