How would you parent this situation?

Tell her you're taking her to the medical tent or for a walk to help with her headache. Tell her that sitting in the tent will make it worse. Then once away from the tent take her to the activities or see some music and just ignore her whining.

I agree with taking any screens, etc off of her and make sitting in the tent as boring as possible.

Agree. Pack up. Go home. Remove all electronic devices from her. Once home she is not allowed to go out or have any devices. She can sit in her room. She sounds horrible. And I would not be giving her any of her birthday gifts till she’d apologised and showed improved behaviour over some days.

What @Whaleandsnail6 said. No way would I let her ruin it for everyone by going home

I would probably pick her up and move her, but if you (understandably) don’t want to do that, I would be clear with get that XYZ severe punishments will happen (and stick to that) unless she moves, because she has no right to ruin everyone’s expensive weekend away.

If she still won’t budge then tag team with your husband to babysit her. Whoever is with her just read and have a beer, don’t entertain her.

You could perhaps say you are going to leave her in the tent, which might get her moving, but one of you would have to watch her from afar if she didn’t, so she knows you will stick to your guns.

This phase will pass but it’s infuriating while it lasts, and I’d come down as hard as you can on it.

What does she intend to do in the tent to pass the time? Whatever it is remove it. She can help pack up while dh take the toddler to have fun. Then you both sit in the car and do nothing... Consequences when you get home.

I agree, I would also start packing up and see if she changes her mind, if not then it's a lesson learnt or she genuinely doesn't feel very well.

I'd be taking her phone or whatever entertainment she is using in the tent and telling her its either sit with absolutely nothing to do in the tent or come for a walk with you.

I would also tell her that she is spoiling the day for me and if she doesnt snap out of it, then she is grounded for the rest of the week once back.

Id let her sulk around the festival but at least you would be there and not stuck in the tent. I'd completely ignore any attitude once out of the tent.

I'd cut your losses and go home. That's the natural consequence of her behaviour. It's not fair for you to have to waste your day by hanging round the tent all day - if she doesn't want to do the booked activities, then she will just have to tag along with whatever you want to do.

If she really is feeling unwell, then home is the best place for her anyway.

DH takes toddler off to keep them amused. You commence packing up.

Give her the opportunity to get over herself once she sees you starting to pack up, but if it isn't forthcoming, then crack on and go home early.

Pack up the car and go. ‘Right we’re leaving then’. No conversations.

Either she’ll start magically start feeling better or it’ll be a lesson to leave her with relatives next time!