The family of four, including their two twins Louis and Lexie, 12, were going to McDonalds as a treat for their children after what they described as a really difficult day at the hospital.
In a public plea to thank the mystery stranger, Chris wrote on Facebook: "If you're the person driving a blue, maybe black, 2015 KIA Sportage through the drive-thru Mcdonald's at Eye Green around 6pm this evening (yesterday, January 12) who randomly decided to pay for the car behind's food... that car was driven by me.
"A man who was taking his disabled child for a treat for being so, so brave at an absolutely horrible hospital appointment today and your completely random act of kindness made our day immeasurably better and you nearly made a 45-year-old man cry.
"We didn't have a chance to thank the person so if anyone doesn't mind sharing my post we'd be really grateful."
Speaking to Peterborough Matters, Chris added: "It was a lovely way to turn a bad day into a good one.
"For the person in front, it may have 'just' been paying for food but to us, it was a reminder that others care for others, and when we are down, there are good people who have our backs.
"It was exactly what we needed at exactly the right time."