Friend has invited herself to our honeymoon

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*namechanged

Some tactful advice needed please!
Husband to be and I have several kids between us in our blended family. Instead of doing a honeymoon with just us, we are taking all the kids away with us for a week on a road trip. My best friend of thirty years (Friend 1) is coming too as she has kids the same age who are really close to mine and our kids all love each other. We were looking forward to chilling out in the evenings to gather whilst the kids played harmoniously.

Friend 2 has just said when I was telling her about the plans that because the place we are visiting is one of her favourite places, she wants to come. She's a high drama person, very controlling and this isn't what I want. Problem is she said it in front of the kids who got excited about her coming so they have been chatting away all week now about the plans.

I need to manage this and do some damage control. She's saying she would pay for herself etc so it's not the money thing. Friend 2 doesn't have kids, and has mental health issues that can be quite overwhelming to manage. She's had a horrible year but she's really hard work and does need managing.
She also doesn't know Friend 1.

It's my own fault but I was so taken aback by her saying she wanted to come along in front of the kids that I agreed in shock, now I'm just thinking this is going to be a nightmare! How best to tactfully manage it without sending Friend 2 into a misery spiral.