The incident occurred at The Plantation mobile home site in West Park Road, Newchapel, Surrey.
The blaze, which killed the 36-year-old mother and her children, aged one and six, broke out at The Plantation mobile home site in Newchapel overnight.
The child has since been taken to the East Surrey Hospital in Redhill. A joint investigation between the police and fire service has been launched, Surrey police said.
A man and another child were treated for burns at the scene, the BBC reported.
A spokeswoman for the fire service said, "The fire was extinguished using hose reels, however, three people - two children and one adult - died".
Surrey police said the scene remains cordoned off while enquiries are carried out into the cause of the fire. Police guards stood at entrances to the mobile home site, which is beside a busy main road.