In the question:
They will be very cold when they go to Scotland next December.
It seems to me that this is a quite certain plan, like one of the examples here: ‘ Cuando voy de vacaciones a Tenerife me hospedo en el hotel Olimpia.’
So I thought the answer should be in the indicative:
Tendrán mucho frío cuando irán a Escocia el próximo diciembre.
But the correct answer was in the subjunctive:
Tendrán mucho frío cuando vayan a Escocia el próximo diciembre.
Why is that? Is it a mistake?