The Alm-Uncle (Grant Towers) and Heidi (Megan Rigby). Photograph courtesy National Children’s Theatre.

Children’s theatre has the license to take the idea of soppy and stretch it to biblical proportions, which enables adults and children alike in the audience to cry with empathetic abandon, as the characters can declare love for one another with the kind of fierce naïve sentimentality that on a grown-up stage would be laughed out the door on cynical tide.

This happens gloriously in the National Children’s Theatre’s latest production, Heidi in a stage adaptation by Francois Theron. Written in 1880 by Johanna Spyri, the story of the girl from the Alps has been loved by generations of young readers – it was screened on South African TV from 1979, in the form of a Japanese anime series – and is understood to be by far the most popular Swiss piece of literature ever published, but a closer look at it reveals a harsh tale coloured with cruelty and disappointment and the nuances of class in 19th century Europe.

Heidi – or Adelheid, as she is more formally named in smart Swiss society – (Megan Rigby) is a hapless child. Orphaned as a toddler, she is raised by her aunt Dete, who is a governess (Emilie Owen), until she’s about five. And then a job prospect teeters the severe woman into dropping her young niece into the context of her uncle, a man who lives in the Alps with only the goats and the landscape as succour (Grant Towers). The uncle, a big, bearded man has a reputation for being scary, one that he honours with aplomb, treasuring his solitude, as he does.

No sooner is the little girl settled in the magnificent Alps, where she gets to laugh and play and imbibe the air and goats’ milk alongside goatherd Peter (Dale Scheepers), to say nothing of exposing a secret part of her grandfather’s heart, a situation under the stern eye of aunt Dete develops for her: to be the companion to a young wheelchair-bound girl, Clara (Caitlin Salgado), in the posh city of Frankfurt, and once again, the child has to undergo an emotional volte face to confront a whole new world and figure out where she fits into all of it.

While the work is coloured by stereotypes – all the governesses are merciless to the point of sadism – the narrative is conveyed with an authoritative directorial pen. The complexity of the tale is handled with wisdom and an intimate knowledge of the theatre’s audience: whilst it grapples with difficult abstract concepts like death and love, immorality and class discrepancy, it does so adeptly, offering the story in clean lines and balancing it with musical forays, which sparkle with sincerity, but never overbalance into too much schmaltz.

Choreographically and musically, this is a large work, which belies, but doesn’t undermine the theatre’s tiny space. The gestures are wide and generous, the songs sung with a bigness of force and the dancing is celebratory: half close your eyes and you can imagine it all happening on a grander stage.

And challenges of the tale itself aside, once again, the theatre presents a delicious young cast headed by Megan Rigby in the eponymous role opposite Grant Towers as the uncle with a secret inner life. The casting of these two newcomers to South African theatre is absolutely impeccable and both rise to the occasion, articulating the bold emotions, yet three-dimensional sense of spirit of both fairly complicated characters with conviction and intelligence.

• Heidi, written by Johanna Spyri in 1880, is adapted and directed by Francois Theron. It is performed by Daniel Fisher, Jana Louw, Venolia Manale, Emilie Owen, Megan Rigby, Caitlin Salgado, Dale Scheepers and Grant Towers, and designed by Graham Brown and Stan Knight (set); Rowan Bakker (musical supervisor); Nicol Sheraton (choreography); and Jane Gosnell (lighting), and performs at the National Children’s Theatre, Parktown until April 12: 011 484 1584.