Without having read the novel, seen the film or either of its two musical adaptations, it may come as a surprise that the entire plot of Whistle Down the Wind pivots on a moment's misunderstanding, and that Cathy – quite a big child – is convinced Jesus Christ himself is hiding in her dad's barn.

But the power of this unshakeable childhood faith is more believable given that this is 1950s rural Lancashire, steeped in Sunday school dogma and a God-fearing lifestyle presided over by a nervy vicar (Bryan Hodgson) and irritable teacher (Danielle Morris). Here, such a miracle might well have seemed real to imaginative children with few outlets.

Russell Labey and Richard Taylor's adaptation of the Mary Hayley Bell novel dates back to the 1990s, and is also based on Bryan Forbes' 1961 film.

The show's success depends on the right level of earnestness, faith and fervour from the village youngsters, who are soon all swept up in the excitement of protecting the fugitive 'Jesus'. And here we have another leap of faith, as in Sasha Regan's production they are all played by adults. Thankfully they manage to roll back the years very effectively.

As Cathy, sweet-voiced Grace Osborn portrays a clever, brave child on the verge of adolescence, experiencing troubling new emotions alongside her natural concern for the wounded Man. Imelda Warren-Green has some lovely comic moments as anxious little sister Nan, while Alex James Ellison brings an eager-faced innocence to brother Charles.

Callum McArdle has a brooding presence as the Man, who's touched by the adoration from the children, and there's also fine work from Kathryn Hamilton-Hall as put-upon Auntie, and indolent Dad Chris Coleman.

Some individual vocal performances are a little thin, so the singing is at its best in the ensemble sections, with the tight musical quartet led by musical director David Griffiths. It's good to see the musicians live on stage, too.

The brick interior of the Union Theatre is a perfect space for the barn where much of the action takes place, and Nik Corrall's simple, draped set works well. He's also done a great job with the dreary post-war 1950s clothing for the children – all thick woollen cardies and droopy socks.

Sasha Regan has staged the set-pieces and whole-cast numbers very effectively, but the ending is sadly fudged, leaving non-aficionados uncertain where the fugitive has gone, or whether he's escaped at all.

Whistle Down the Wind is very much a period piece, and this is an engaging and committed performance from the company at the Union Theatre.

Whistle Down The Wind runs at the Union Theatre until 21 February