F7 and F9, strange bump in hair

Hi everyone,

My partners daughters, aged 7 and 9, always develop a strange bump when they go to their family home in the mountains. It happens after one day, continues growing and goes away by itself once they leave. There is no change in washing powder, sheets, pillows or shampoo (they don’t stay long enough to wash their hair). He’s asked pharmacists and they had no idea. They go every year and the youngest started getting them last year. It’s their annual family reunion and none of the adults or elders have it. It’s itchy but not painful. They play a lot under a linden tree, but other than that we have no idea what could cause this. Any takes ?

Thanks !!

Pic in the comments