You’d need to have the humbuggiest of hearts not to be charmed, even just a little bit, by this family Christmas movie – lavishly adapted, no expense spared, from a kids’ novel by Matt Haig. It begins in modern-day London, with Maggie Smith as the Mary Poppins-ish great-aunt to a trio of siblings. Their mum has recently died, so no one is feeling festive when Aunt Ruth begins telling them a bedtime story on Christmas Eve.

Her fairytale is a Santa Claus origins myth, about a boy called Nikolas (played by delightfully urchin-faced newcomer Henry Lawfull). He lives in a forest in Finland with his poor woodcutter dad (Michiel Huisman). They are so poor that when the dotty king (played by Jim Broadbent with endearing spoiled-child petulance) offers a reward to anyone who can bring hope to the land, Nikolas’s dad goes off in search of Elfhelm, the fabled kingdom of elves.

Enter Kristen Wiig, giving it the full Mrs Twit treatment as child-hating ’orrible Aunt Carlotta, who arrives to look after Nikolas. The unspeakable thing she does to his only toy – a doll carved out of an old turnip – is a genius stroke of kids’ writing. So young Nikolas runs away to find his dad, taking with him his talking pet mouse (voiced by Stephen Merchant). On his adventure, he acquires some dead giveaways as to his future career: a red hat with a white bobble, and a reindeer friend called Blitzen. When he finally catches up with his dad, there are some honest unglossed truths about how parents disappoint their kids – and the film has semi-unsentimental things to say about grief too.

Still, above all this is A Christmas Movie, made for audiences from four to 94, little ones watching with nan on Boxing Day in front of the telly, everyone a bit drowsy on Quality Street. Maybe because of that, it feels kid-gloves at times: big-hearted and entertaining, but possibly lacking a little fun or oomph. A lovely warming film, though.

• A Boy Called Christmas is released on 26 November in cinemas and on Sky Cinema.