Two boys aged 13 and 14 have died after a “serious incident” at a property in Sheffield, police say.
Four other children aged 11, 10, three and seven months are being treated in hospital and are “conscious”, police added.
A man, 37, and a woman, 34, have been arrested on suspicion of murder.
Police are not looking for anybody else.
Cause of death has not been established and a post-mortem is due to be carried out later on Friday.
South Yorkshire Police said “family have been informed and are receiving support from specially trained officers”.
There is “no wider risk to the community”, the force added.
Officers were called to the property at about 7.30am.
“We are working hard to ascertain the exact circumstances surrounding this incident,” said Detective Chief Inspector Simon Palmer.
“We will remain in the area this afternoon and into the evening carrying out enquiries and providing reassurance.
“I appreciate the events of this morning have caused concern locally but I’d like to reassure you that there is no wider risk to the community.
“To allow the investigation to progress, I would please ask that people are mindful of what they are posting on social media and the potential distress this may cause.”
Six children have been taken to hospital after a “serious incident” at a property in Shiregreen in Sheffield, police say.