Are children aged 17 and under allowed to travel with European Sleeper if they are accompanied by 1 person?

A child aged 17 and under may travel with European Sleeper if they are accompanied by 1 person. One adult may travel with a maximum of 4 children in their care. The child must have written permission from the parent(s) or guardian. You will find the form for written permission here: 

https://www.rijksoverheid.nl/onderwerpen/reizen-met-kinderen/documenten/formulieren/2014/02/06/formulier-toestemming-reizen-met-minderjarige-naar-het-buitenland.