For au pairs, entry visas are issued that are valid for the entire Schengen area.

This is important because international flights do not always lead directly to the country of destination.
A separate transit visa would therefore be required for each transit country.
Please make sure that the first airport of entry is in a Schengen country.

Modern international airports in Europe are divided into a "Schengen area" and a "non-Schengen area".
Once in the Schengen countries, the AuPair enjoys freedom of movement throughout the Schengen area.