Music: Childish innocence, gallows humour: Meredith Oakes on the children's protest opera created amid the horrors of a Nazi concentration camp

IN 1941, the Czech Jewish composer Hans Krasa rescored his 1938 children's opera, Brundibar, for performance in the Nazi concentration camp at Theresienstadt. It became a ghetto favourite, with over 50 performances. Surprisingly, it wasn't banned, unlike Viktor Ullmann's Theresienstadt opera The Emperor of Atlantis: Krasa's piece, artistically less bold, is no less politically outspoken.

Last week, Mecklenburgh Opera gave it its British premiere, part of the South Bank's Czech Festival, in an English-language version (by John Abulafia) excellently realised by the New London Children's Choir in a double bill with Ullmann's more familiar Emperor.

Brundibar is about a cheerful village dancing to the tune of an organ grinder who permits no music but his own. Two children arrive, wanting to sing for money to buy milk for their sick mother. They are prevented by the forces of law and order. Nature comes to their aid: village animals and children chase the organ grinder, and the people, their kindly feelings dawning, give money for the milk. A final chorus praises the power of love against tyranny (though, in the action, it is the fangs of the local dog that prove decisive).

The Mecklenburgh Ensemble brought lovely flexibility and lift to Krasa's delicate wrong-note nursery-tunes. Christopher Baugh's skewed black platform, the basic set for both works, sported papery cottages, flowers and fences: the village sparrow entered centre stage on a swing, RAF insignia decorating her blue wings. Skipping, smiling and rosy cheeks were compulsory: the very musical cast of children threw themselves energetically and with great good nature into their ironic task, and their singing was beautiful.

Playful tenderness was an achievement in the climate of the times: impressive in Brundibar, it becomes truly extraordinary in The Emperor of Atlantis, where it coexists with a complete understanding of the inmates' true situation. A flu-struck cast delivered all the jokes ('How can you sing?' - 'I always sing') and all the grace of this sublime cartoon, which should always stay in the repertoire. Gwion Thomas gave full trouper value as the Loudspeaker, Brian Bannatyne-Scott was very gentle as Death, Anne Manson an endlessly resourceful, sensitive conductor.