Fire fighters were called to Brothers Ink, in West Road, Shoebury, Essex, at 5am on Friday to tackle a blaze that caused £25,000 damage.
Families nearby were evacuated while crews put out the fire.
Pc Paul Leger said: "Whoever did this was completely reckless and showed total disregard for these people, some of whom were young children.
"We dread to think of the possible consequences had the firefighters not extinguished the blaze sooner."
Anyone with information should contact the police on 0300 333 4444.