Civilian aviation in the early months of ww2 in europe

Context:
So my grand father told me that during the beginning of ww2 he was sent to denmark by his parents to escape the war and he took a plane with other children (then he was sent back to france when denmark was invaded).

But what made it possible (Was the air space “neutral” for civilian aircrafts ?) ? What was the policy for civilian aviation during this time ?

Thank you for your meticulous work =)

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