Chief Inspector Vicks Hayward-Melen from Avon and Somerset Police said post-mortems will now take place to establish the children’s cause of death. They are likely to be completed by the end of this week.
Dan Whitehead, Sky News’ West of England correspondent, said it is understood the seven-year-old was a pupil at Sea Mills Primary School, which decided not to open until the afternoon on Monday, the first day back after half-term.
He said a local playgroup also delayed opening following the “tragic incident”.
Chief Inspector Hayward-Melen said police are treating it as an isolated incident and there is no ongoing threat to the wider community.
“The death of such young children is a great shock to the whole community, and this incident has had a profound impact on all of us in the police,” she said.
“We’ve been humbled by the community response to this tragedy. At a time of great sadness and disbelief, we’ve seen spontaneous acts of care and support.
“Within hours of the incident occurring, a local church was opened up for people to gather and mourn. This is what being in a community is all about and we’d like to thank all those who continue to offer this important and selfless service.”
She said detectives from the Major Crime Investigation Team are in the early stages of what she called a “highly sensitive, complex and thorough investigation”.
The Independent Office for Police Conduct (IOPC) has been alerted of the incident because there was “prior police contact earlier this month”.
An IOPC spokesperson said earlier on Monday: “We have requested a paper referral with further information about the prior contact and once received, we will assess it to determine whether further action is required from us.”