Connor Bradley, 6, began jumping on the bed with his four-year-old sister Chelsea Piggford after their mum left the room.
Just seconds later, mum Vikki Bradley, 24, heard screaming from Chelsea and burst into the bedroom to find Connor had gone.
She looked out of the window of her home on the Tudhoe Moor Estate, near Spennymoor, to find her son lying in a pool of blood on the concrete block paving below.