Nightmares: Why do they suddenly start in young children?

Recently my youngest daughter, Little Miss George (4), had great fun hosting her elder sister, Princess Peppa (6), for a run of sleep-overs in her bedroom. We had a mattress set up on the floor of Little Miss George's bedroom, cuddly toys and all, for Princess Peppa to sleep in. She slept in there for roughly a month.

Why? Because we had a leak in the ceiling off Princess Peppa's bedroom stemming from last years winter of double thick level snow covering our roof. Without getting into too much detail (you can always tweet me if you're interested), previous owners of our house had taken out the chimney breast in Princess Peppa's room and left the chimney above. It wasn't well supported, and VERY tall, so we had that taken off too and capped. Then the snow came down last year and sat on the capped roof for weeks, slowly getting in behind the render as it hadn't been capped by the workmen correctly (with overlapping edges) so the water ran down into the loft space and ultimately into her ceiling.