Vandals set fire to a small building behind a nursery in Heanor on Sunday morning.
Crews from Ilkeston and Heanor were called to Cottage Day Nursery on Mansfield Road at 5am.
The alarm was raised when smoke was seen issuing from behind the nursery and smoke alarms were heard. Two firefighters in breathing apparatus used one hose reel jet and a safety jet were used to put out the flames.
Police were informed as the fire was believed to have been started maliciously. The fire was out by 5.35am and crews damped the scene down and left at 5.50am.