Strong winds were blowing in the area, hampering efforts to extinguish the blaze.
Authorities said the summer camp with around 400 children was being evacuated.
Wildfires fires are common in Greece during the country's hot, dry summers. In 2018, more than 100 people died when a fast-moving forest fire swept through the seaside settlement of Mati, northeast of the Greek capital, Athens.
The fire department said it would be carrying out patrols by aircraft on Friday as well as joint ground patrols with the policyeand army in areas where the fire risk was elevated due to weather conditions. The areas include the wider Athens region, other parts of mainland Greece and several Greek islands.