The two children, aged 13 and 14, were crushed by falling trees while sleeping in tents and around 20 other children were injured when a freak storm hit a campground for Boy Scouts and Girl Guides with 140 youngsters in the northern village of Suszek.
A woman died when a tree hit her house in the northern village of Konarzyny while a man sleeping in a tent in a nearby village also perished after being hit by tree brought down by high winds.
An unusual heatwave saw peak temperatures soar to the high-thirties Celsius (around 100 Farenheit) across Poland in recent days, but milder weather is expected this weekend when the mercury is forecast to dip to the mid-twenties in most regions.