Well, it's certainly a first in kiddie cries of participation at panto time: "Don't sit on the confabulator!"
Those silly old Brothers Grimm do just that - a confabulator is a story machine - while searching for their next tale and, as a result, they fall inside, rather like the wicked witch shoved into her own oven at the end of the gluttonous child abuse she's perpetrated.
We often say children are good enough to eat, but only the Grimm witch goes so far, having lured the unwanted siblings into her candy cottage. It's a terrible, genuinely frightening story and it was given the right sort of lurid nice but nasty treatment by Kneehigh a few years ago.
Katie Mitchell's approach at the National is to frame it in some mildly funny Grimm Brothers knockabout - Justin Salinger and Amit Shah in top hats chase flying stories with butterfly nets - and decorate it with some low-tech puppetry and the wheezy exhalations of a musician (John Paul Gandy) seated at a keyboard with eccentric add-ons.
This ambitious set-up is never quite matched by the intensity of performance or the quality of Lucy Kirkwood's script and Paul Clark's dissonant music. Something about Kate Duchêne's witch (doubled with her modest manipulation of the children's dog, Jennifer) fails to catch fire, and the title characters of Dylan Kennedy and Ruby Bentall are annoyingly anodyne.
The Grimm actors double as the appalling parents on the edge of the Black Forest, with Salinger also doing a good turn as the flying man-bat, Stuart. The programme comes with handy instructions on how to cut out and keep your own puppet bat and dog, but mutinous young children might have to be assuaged with a visit to Sooty or Stickman.