Rick Webb, 30, said after various operations, his son, Oscar Webb, has had to have his right eye removed.
Oscar was playing in a neighbour's garden while his dad watched family friend Simon Evans, 29, fly his £300 quadcopter device with a remote control.
However, the quadcopter clipped a tree as Evans was trying to land it and spiralled out of control, hitting the toddler in the face.
"I saw the propeller speed up and I saw the blade spin right through his eye," said Webb. "I saw it go in and a bit of his eye landed on his t-shirt. It was just horrifying."
"He screamed through the shock and so did I. The sound was terrible, like nothing I've heard before," said Webb, from Stourport-on-Severn.
Webb phoned for an ambulance and paramedics immediately came to the house in Worcester on 10 October.
Oscar's mum Amy Roberts said: "When it happened I was taking my other son to a birthday party and I got a phone call from Rick telling me Oscar was hurt.
"Luckily, I was only round the corner and I got there before the ambulance.
"As I arrived, Oscar was crying and screaming and his dad was very upset and really struggling to calm him down.
"What I saw, I can still see it now, and what I saw or what I thought I saw was the bottom half of his eye and it's the worst thing I've ever seen.
"I just hoped and prayed all the way to hospital that what I saw wasn't true and wasn't real."
However, the damage was too severe and on 2 November surgeons were forced to remove the injured eye at Royal Stoke University Hospital.
Oscar will require several further operations to mould and fit a prosthetic eye.
He is still able to see through his left eye, but his parents think life will never be the same for their family.
Webb said: "I don't know how I feel now to be honest. I've got to hold it together for the sake of my family.
"He's a strong lad and he's got two parents who love him. We're a very close family and he'll always have people there for him.
"People don't realise and understand how dangerous drones can be.
"I've got one myself but I'll never fly it again. People should wear eye and neck protection because those propellers can be deadly."
Roberts, a 28-year-old care worker, added: "After the operation the doctors told me he might not have lost the sight in that eye.
"The following month they told me they had decided to remove the eye because there was no chance of saving it and it might cause future problems for his good eye.
"It was so hard to hear. I went there with so many questions and I didn't even get to ask the first one before they told me the news.
"After the operation he's been a lot better in himself, he's getting back to the way he was before.
"I'm so proud of how he's coped with this. He's amazed me so much."
Roberts continued: "I honestly don't think we'll ever get used to it. This has devastated the whole family.
"He's having another operation soon to have a mould taken for his prosthetic eye. Then he'll need more surgery to have it fitted but that won't be the end of it.
"For the rest of his life he'll have to remove and clean it, he'll have to go to hospital appointments and have it checked and taken out and polished."
Roberts, who has three other children - Ashleigh, nine, Thomas, five, and Lacey, four - with partner Rick, says she doesn't hold a grudge against the Evans for causing her son's life-changing injury.
She said: "It was a completely freak accident and I know [Simon] is suffering himself. He's a very close friend and loves the children.
"He hasn't flown the drone since and he says the thought of it makes him physically sick.
"I know there are a lot of experienced drone flyers but a lot of people aren't aware of the dangers.
"We didn't think in a million years something like this could happen.
"I want to get it out to every parent, whether they're flying drones in their gardens or in the park, they need to take precautions.
"These are not simple toys.
"There is a risk in anything in this world but if that risk can be reduced it's got to make a difference.
"A woman messaged me on Facebook recently to say her son was cut on the face by a drone. They're not safe for children to be around."
Eye specialist Faye Mellington, from the Birmingham and Midland Eye Centre, said: "We knew straight away the outlook for Oscar's vision long-term was extremely poor.
"'I have seen a lot of ocular injuries, but never in someone so young, and I've not seen one from a drone.
"That said, given their popularity and the common use, it's inevitable that we'll see a lot more.'"
The Civil Aviation Authority has guidelines for flying drones safely, which are included in drone packaging.
There will be a public consultation on a licensing and registration scheme for drones before a Government strategy is published next year.