The deceased were identified as three-year-old Dishu and her 50-year-old grandmother Roopwati, residents of Maadipur in west Delhi.
A senior police officer said that all the three injured were taken to Sanjay Gandhi Memorial Hospital where the girl and her grandmother was declared brought dead.
The police said that the three were heading towards home when the accident occurred at around 1 pm yesterday.
"They were near Metro pillar number 242, close to the West Paschim Vihar Metro station here when the speeding truck hit the scooty from its front left. The impact of the hit was such that all three fell off the scooty and received serious injuries," the official said.
The official said the accused was identified as Gopal Singh who was apprehended by one transporter, Balvinder Singh.
Singh was driving his Swift car and saw the truck driver fleeing with his vehicle after the accident, he added.