Officers were called by the ambulance service just past 5pm on Tuesday after a report that a dog had attacked a child in the back garden of a property in Broadlands, Netherfield.
It was confirmed shortly after that the child had died at the scene.
The dog has since been “humanely destroyed” following the incident.
No-one else was hurt and the child’s family is being supported by specially trained officers.
No arrests have been made in connection with the investigation.
Superintendent Matt Bullivant said: “This is an absolutely tragic incident in which we believe that a child has been killed after being attacked by a dog.
“Clearly we are in the very early stages of an investigation, and it would be wrong and unhelpful to speculate as to the exact circumstances of this incident at this point.
“However, in order to ensure public safety, a decision was made earlier this evening to destroy the dog involved, and this has been done humanely.
“I understand how much of an impact this will have on the community and on the wider public, and people can expect to see a large police presence in the area this evening and beyond while our investigation continues.
“I would like to reassure people that there is no reason to believe there is any danger to the wider public at this time.
“Anyone with concerns should feel free to approach our officers and ask questions, but I urge people not to speculate about the circumstances, especially on social media platforms.
“I also understand that this case is likely to attract attention from the media. With this in mind I would like to ask that the privacy of the family involved is respected at this very difficult time. It is impossible to imagine what they must be going through at the moment.”
The force urged anyone with information to call 101 quoting reference number 20230131-1546 or to call Crimestoppers anonymously on 0800 555 111. Reports can also be made to police online.