It's an occasion that might make even Mr. Grumpy crack a smile. The colourful Mr. Men children's books are celebrating their 50th birthday.

The popular series has now entered its sixth decade in 2021, surviving its British creator Roger Hargreaves thanks to the dedication of his son.

"It is an amazing fact that we've reached half a century, it's a very long time for a series to be so successful," Adam Hargreaves told AFP.

The books' enduring popularity after the first story's publication in 1971 is an indication of "how strong my father's idea really is", he added.

The younger Hargreaves played no small role in the genesis of the Mr. Men and the Little Miss series that followed, whose characters are named after the emotion or behaviour they embody.

It was his own childish question -- "one of those impossible questions that children like to pose to their parents," he said -- that prompted his father to draw the first Mr. Men character.

"My question was: 'what does a tickle look like?'" Hargreaves explained.

In response, his father drew a little orange man with a blue hat and arms of incredible length. Mr. Tickle was born.

The character became a book, and that book, in turn, became a runaway success, even leading to a television series narrated by "Dad's Army" actor Arthur Lowe.

Over the following years, Mr. Tickle was joined by a cohort of friends, including the perpetually smiling Mr. Happy and the accident-prone, heavily bandaged Mr. Bump.

Adam Hargreaves took over the Mr. Men series of children's books, created by his late father, which is marking its 50th birthday
Adam Hargreaves took over the Mr. Men series of children's books, created by his late father, which is marking its 50th birthday Mr. Men Little Miss, Sanrio / Handout

The Little Miss series followed 10 years later.

Since their inception, some 250 million copies of the small square books have been sold in 30 countries around the world, from Britain to China, and translated into 17 languages.

Hargreaves said that his father had always been ambitious about the books and saw the potential of the series.

"But in terms of whether he thought it would have lasted for 50 years I don't think he could have imagined that, half a century on," he said.

The success of the Mr. Men and Little Miss series was not always assured, however, and could have come to an abrupt end when Roger Hargreaves died suddenly at the age of 53.

Following his father's death, Adam, who had always loved to draw, decided to take on the books himself.

"One of the most difficult things was actually taking on board the idea of creating something," the 57-year-old said.

"There was a kind of reluctance on my part to actually create anything new. And I just saw it as my dad's idea," Hargreaves added.

Creator of the Mr. Men series, Roger Hargreaves had his first book published in 1971
Creator of the Mr. Men series, Roger Hargreaves had his first book published in 1971 Mr. Men Little Miss, Sanrio / Handout

But he did go on to create new characters and finds inspiration for them often in the world around him.

Fans are currently being asked to vote on for two new characters to join the gang in Happyland.

Hargreaves still finds himself returning to his father's books when he feels stuck for ideas.

"I think a lot of people might have thought it was kind of quite difficult to do emotionally," he said of following in his father's footsteps.

"But actually, I found it quite comforting, because it sort of brought me closer to him."

There are some things Adam refuses to change: the "unique style" of his father's drawing, the interior decoration -- "definitely stuck in the 70s" and the "old-fashioned" rotary telephones.

He believes the strength of the Mr. Men and Little Miss books lies in their timelessness.

The characters are "based on a little bit of us, our emotions and our characteristics. And obviously, those don't date", he said.

"Children are looking at the Mr. Men, Little Miss series in exactly the same way as they were 50 years ago," he said.

"So, conceptually, the idea doesn't need to be updated or modernised."

The core ideas of the books may have remained the same but the series has also moved with the times.

In recent years, Hargreaves has put characters from the BBC's popular science fiction series "Doctor Who" into the books and members of pop group the Spice Girls.

During the Covid-19 crisis, the books have also played an important role.

"Through this pandemic, Mr. Men and Little Miss have helped children understand how they feel," he said.

"Mr. Grumpy will show them what anger is and show them how to deal with it, in a very kind of funny and amusing way," Hargreaves added.

A father now himself, Hargreaves said he had no plans at present to put down his brush.

But he hopes that when retirement does beckon the series might stay in the family.

"You never know, there might be another generation of Hargreaves drawing and writing Mr. Men in 50 years time," he said.