A five-month-old baby was killed after a van mounted the pavement and crashed into his pram.
Louis Thorold died after the van left the road, having collided with a car on the A10 in Waterbeach, Cambridgeshire at about 15:30 GMT on Friday.
His mother Rachael Thorold, 36, who was pushing the pram, is in hospital.
Louis's father Chris Thorold said in tribute to his son: "You were only with us for a short time, but you made us the happiest people in the world."
Police said Ms Thorold, who is currently on maternity leave from her role as a planning policy manager for Elmbridge Borough Council in Surrey, was in a critical but stable condition.
The Renault Master van driver and the driver of the car, a grey Mazda 2, suffered slight injuries.
Mr Thorold, the finance director of Marshall Aerospace, said he "cannot describe how heartbroken we are that [Louis is] gone".
He added: "You are such a happy little boy and your joyful smile and laughter will never leave us.
"Your ma-ma is fighting to stay with us and we are praying that she will make it through.
"You will always be in our hearts and do not worry, I am taking care of all your animals.
"We love you so much and we know you will now be safe with granny, grandad and great grandpa in Heaven.
"Take care my little Lou, sweet dreams."
A Cambridgeshire Police spokeswoman said inquiries were ongoing and no arrests had been made.
The force has appealed for witnesses or dashcam footage.
Det Sgt Mark Dollard called it an "extremely tragic and sad incident" and sent the family "our deepest condolences".
"While it is too early to say how the collision happened, I want to remind all road users of their responsibilities to drive with care," he added.