Fiona and Wesley Ratcliff, from Woodhouse Park in Wythenshawe, Manchester, had spent the last two weeks transforming the summer house into a safe play space for their seven -year-old s on Harvey, who suffers from Phelan-McDermid syndrome, (22q13) a rare genetic disorder.
They had planned to show Harvey the sensory room for the first time on Christmas Day, but instead they discovered it had been destroyed in an arson attack.
Wesley Ratcliff, 32, said: "Everyone was devastated. We had family come down from Scotland. They had put in the money towards helping us build it. They had all come down to see his face.
"We are all in shock. The consequence with what could have happened; it being so close to the house - I think we are lucky."
A guinea pig hutch next to the summer house had also been set alight, but the two guinea pigs escaped unharmed.
Mr Ratcliff, who is a support worker with autistic adults, said: "We bought the summer house at the beginning of the month, and the idea was to get it up and ready for Christmas.
"We painted it, put the flooring down, put the lights in, decorated it with pictures and painted it sky blue because Harvey is a Manchester City fan. It was just somewhere where he could go when he was having a hard time. I had put the final touches in on Christmas Eve."
But yesterday morning Mr Ratcliff found that the summer house had been destroyed by a fire.
He said: "The disco lights and a digital radio had been stolen. We had put soft floor mats inside which were completely burnt by the fire. The bean bag chair and a little stool and easel table were all destroyed as well.
"My opinion is that we have not been targeted, but that it was just a random attack. It was probably just some drunk teenagers, a one off. My wife is really upset and she is scared about being alone with the kids in the house."
Detective Inspector Jane Curran said: "This truly is heartbreaking, the family, like every other family across Greater Manchester, were really looking forward to showing their son his Christmas present when they woke on Christmas Day.
"Whoever did this really has no morals whatsoever.
"It is a disgusting and cowardly crime and I really would like anyone who may have seen anything or who has any information about this to contact us.
"Thankfully we think that the weather overnight extinguished the fire otherwise we may have been dealing with a much more serious incident yesterday."