A community is being urged to help police find two youths, who may be as young as 12, who broke into a flat and held a knife to a baby's neck.
They brandished kitchen knives at the four-week-old girl and threatened her 25-year-old mother until they were given £20 and a few US dollars.
A second flat was then broken into, also in the Grangetown area of Cardiff.
Local people are being asked to help trace those behind a crime described as shocking and terrifying.
Det Sgt Stuart Adams, said the mother suffered "horrendous fear" during a "nasty" crime.
"The baby is fine, being four weeks old she wasn't put in fear, but the mother was terrified"
"I imagine the people of Grangetown will be frightened about such an incident happening in their area but I would like to reassure them that we will catch these people," he said.
"I would hope the people of Grangetown would come forward with the names of anyone who may have any information on who has committed these crimes."
Police said the mother handed over the money she had in her flat after the youths threatened her and her baby at about 1830 GMT on Sunday.
One of the young burglars then stayed with her as the other raided a second flat in the same building in Clive Street.
It is believed a 32in LG plasma screen television and a large amount of money was stolen from the second property.
"As a society we ask ourselves why children as young as this are violent"
Det Sgt Adams said inquiries were under way and warrants had been executed in an attempt to get some of the stolen property back, and he appealed to anyone who might come across the TV to report it to police.
He added: "The baby is fine, being four weeks old she wasn't put in fear, but the mother was terrified - they threatened the baby and they threatened to kill her."
Councillors in Grangetown also condemned the burglary.
Councillor Francesca Montemaggi said: "It's shocking. As a society we ask ourselves why children as young as this are violent. This shouldn't happen.
"There's a strong community feeling [in Grangetown] so I think it is a big blow to people that something like this can happen."
The two youths are described as between 12 and 14 years old and wearing hooded tops.
One of the two is described as white with blond hair.
He was wearing a grey hooded top with red lettering, black jeans and trainers.
The second youth is also white and was wearing a white top with the hood up.
Anyone with information is asked to contact Cardiff Central CID on 02920 222111 or Crimestoppers on 0800 555111.
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