How do you uninvite someone who's invited themselves?

I was wondering how to uninvite someone who's invited themselves to a family holiday? Both my wife and I were planning to escape to the countryside later this year and our friend ended up inviting herself along with her two kids. Since then we've been wondering if it's such a good idea for a number of reasons but when it's been brought up in conversation she's managed to play on things and avoid getting told she can't come. So now we're left feeling a little cornered into taking them along when it was meant to be a romantic break away. How can you let someone down gently or should it be done quickly and sharply like a plaster?