Short stories written by children are as likely to introduce you to a “shrinkiniser” machine or the “Land of the Woopa Woopa” as they are to feature Jessie J or Justin Bieber.

Older children are more likely to write about terrorists and anxiety, while younger ones write about cupcakes, ladybirds and unicorns. And though many children embrace complex words and spell them correctly, young people of all ages struggle with apostrophes.

These are some of the findings of a new academic analysis of children’s creative writing released this week. Lexicographers from Oxford University Press said the study of 74,000 entrants to a national short story competition demonstrated the impressive imagination and literary skills of a generation of under-14s.

But it highlighted the perennially sticky problem of the apostrophe and the difficulty many children have spelling some common words. The rise of American English was also identified, with words including “cupcake”, “garbage truck”, “candy” and “sidewalk” featuring in many of the submissions.

Analysts were most struck by the inventiveness of the stories. One child wrote confidently about a “zaporator” (a machine that shrinks planets), while others introduced readers to the “pandaroo” (half panda, half kangaroo) and the “Wackadoodle maniac”.

Imaginative similes, including “as saggy as a baboon’s bum” and “as puzzled as a baby doing proper fractions”, also amused and impressed academics trawling through the stories.

Current events and celebrity culture featured, with David Cameron appearing in 52 stories. Children’s author Jacqueline Wilson and Barcelona footballer Lionel Messi were among the well-known people most often namechecked.

Using special software, researchers were able to isolate distinct trends along the lines of gender, age and geographical location. So while the most commonly occurring keywords in girls’ writing were “fairies”, “pony” and “girls”, the keywords most frequently used by boys were “sniper”, “penalty” and “Sgt”.

Words such as “baddies”, “pirate” and “yum” were among the top 10 keywords for the under-10s, while older children made frequent use of words such as “depression”, “anxiety” and “oblivious”. Researchers were surprised by children’s ability to spell unusual words such as “pulchritudinous” at the same time as being stumped by “can’t”, “does” and “doesn’t”.

The most common noun across all entries to the competition was “door”, used 67,783 times because of its usefulness as a plot device, researchers suggested.

“At a microscopic level, children’s use of language is robust and imaginative,” said Andy Stanton, a competition judge and author of the Mr Gum series of children’s books. “They know the value of a well-chosen word and the power of an original image.”

Jane Bradbury, an English teacher at King’s Norton Boys’ School in Birmingham, who is also a lexicographer and worked on the analysis, said: “From a teaching point of view, it is really encouraging to see what a broad vocabulary children have got, and the effort they are going to to get that right. But it is also really useful to see what kind of mistakes are being made.”

Oxford University Press hopes the database of children’s writing will help it prepare a children’s dictionary and allow it to follow changes in children’s language over time.

The researchers analysed entries to BBC Radio 2‘s 500 Words competition. The six winners were due to be announced today in a live broadcast from the Hay literary festival.

ABSURD WORDS

Fantasy gadgets from the children’s short stories:

Fingerlaser

Electrospecs

Zaporator (shrinks planets)

Shrinkiniser

Electrostone (disables electrical circuits)

Hologrammail

Character names:

Wackadoodle maniac

King Drunkenstein

Prince Roastbeef

Difficult words:

Pulchritudinous

Nefarious

Pyramidal

Ominous (used to describe a character’s goatee beard).