Guests at the party were witness to what appeared to be a "targeted attack" on Zainab Kalokoh, a 33-year-old health worker originally from Sierra Leone. But she may have been a victim of mistaken identity.
The claims emerged as police charged two boys, aged 14 and 16, with the murder on a notorious estate in Peckham, south London, on Saturday night. They were also charged with robbery.
Another two men arrested yesterday, aged 19 and 33, were released on police bail last night to return for questioning in five weeks pending further enquiries.
Ms Kalokoh was holding the six-month-old baby - her niece - when a masked gang stormed the party at a community hall. The baby had just been handed to Ms Kalokoh by its mother, also called Zainab. The mother was moving among the 100 or so guests, including many children, and by the time of the attack she was at the other end of the hall.
Another guest, a Jamaican-born woman, who lives on the Wood Dene Estate where the attack happened, said: "Nobody deserves to die like that. It was an execution. It was an assassination. It was horrible.
"My boyfriend saw them coming. They were wearing balaclavas. They seemed to be clocking the place before they came in. You do not expect something like this to happen with all these people around, including children. There was shock. People could not believe it."
Ms Sillah, also from Sierra Leone, described the terrifying moments when four men "calmly walked in" to the hall at around 10pm. "They wore black facemasks. You could only see their eyes. They fired in the air three times ... I thought it was just a joke but, when I could smell the firing, I thought, 'Oh my God, this is for real'."
The gang then stole guests' handbags, mobile phones and wine before fleeing on foot.
"I ran to Zainab. The baby was still in her hands. There was blood pouring out of her head. I thought: 'My good God, what is happening?' I was shouting: 'Zainab! Zainab!' I was squeezing her head. We were trying to keep her conscious but I think she was dead," Ms Sillah said.
Yesterday morning, detectives were searching for evidence but their investigation will be hampered as no CCTV cameras overlook the hall. The building is protected by spiked metal fences and is on a square overlooked by the estate's tower blocks and a 200ft high incinerator.
Meanwhile, a picture emerged yesterday of the victim as a hard-working mother of two who arrived in the UK 10 years ago and lived in a rented flat in Stratford, east London. She has two sons: Abdul, eight, who lives in the UK though not with her, and another aged between 12 and 14 who lives in Sierra Leone with his grandmother.
One friend, a Jamaican-born man who gave his name only as Raymond, described her as a peace-loving and hard-working woman whose dream it was to start her own restaurant with her boyfriend.
"She was like a sister to me," said Raymond, whose partner lives in the basement flat of the four-storey housing association property in central Stratford.
"She was really friendly and a peaceful person who I can't imagine had any enemies. I would see her every day and I helped fix her car and put up shelves and was going to paint her flat. She had plans to move with her boyfriend to Gambia where I think he was from and open a Caribbean restaurant."
He said that she lived alone in her one-bedroom flat and was visited by her Gambian boyfriend and her sister but worked long hours, including night shifts, at a care home.
He last saw her on Friday evening as her boyfriend dropped her off outside the flats in his car and she kissed him goodbye. "These people are cowards," he said. "How can they do such a thing?"
Another friend and colleague from her former workplace, a care home in Sydenham, south-east London, said: "Zainab was a very, very hard worker. She worked there full time and loved her job.
"She was very, very friendly and I remember she also used to do hairdressing for friends, although she didn't charge for it, it was only as a hobby."