"Vysvětlete tamhletěm dětem, že tady zpívat nemůžou."

Translation: Explain to those children that they cannot sing here.

[sebast1ans]

Spisovný tvar je „nemohou“.

[VladaFu]

Tvar nemůžou je hovorový. Hovorová čeština je součástí spisovné češtiny. Korpus navíc poskytuje více než dost dokladů i z oblasti psaného slova.

[Programen]

Stejně jako "tamhletěm", mimochodem :D

[RichardBul911825]

Why “nemůžou” rather than “nesmějí”? Is it the same, or is it different?

[VladaFu]

nemůžou - cannot

nesmějí - must not

[Clive_Alive]

Is the original Czech sentence wrong? It would make more sense with nesmeji instead of nemuzou.
It makes little sense that children literally, physically, cannot sing somewhere that someone is capable of explaining it.
It does make sense that they must not sing somewhere, e.g. in a graveyard.

I reported it as "other"

[VladaFu]

The sentence is correct. Perhaps it is just not convenient or just ethically/morally wrong. Nesmějí means it is against someone's order or against a rule or law.