Of course they have to make a big thing of some 'secret holiday' because Esme can't have anything resembling a 17 year old's birthday. No party, no friends, no going out, no nothing.
Surely if she wanted to go on holiday for her birthday, it wouldn't be a secret, it would be her choice and she'd prefer to be with friends or at the very least Isla and Isabelle, not the whole lot of them as usual. A holiday is hardly special when they seem permanently on holiday.