She wraps up in the seaweed blankets.

Hogwarts will agree that next time merpeople are wronged merpeople can be consulted on the justice process, sure, but these children were already whipped and docked points and given detention and given extra detention when the complaints of the merpeople were conveyed and all their classmates are angry with them because they can't go near the water and they will not be handed over.

There is some confusion over what whipping somebody entails and whether humans find this deterring and how it is conducted. 

Merpeople take Karen to the surface to refresh her Bubblehead charm.

She recasts her Bubblehead charm and tries to do a warming charm too but can't finish the motion before they snatch the wand back. "I'm freezing!" she screams.

They couldn't be sure it was a warming charm and not something violent. 

Back underwater they go.

Timothy has Minor brought in and tells everyone else to muffle their ears and tells Minor Sonorus -

And Minor yells at the wall that they should please please let them send people who can make sure Karen's okay it'll be a disaster if she's hurt.

Miranda listens through her Herbology earmuffs to the gibberish.

Is that Minor -?

Yep that's Minor yelling really loudly in Mermish!

The merpeople will give Karen any not-a-wand things she wants, what does she want.

She really really needs to be warmer than this seaweed is capable of she's losing feeling in her toes.

They can take her to a sunnier patch of water.

It is FEBRUARY IN SCOTLAND THAT WON'T CUT IT.

- she can do a warming charm if she does nothing else. If she tries to do anything else they won't even give the wand back for the Bubblehead charms.

She does a warming charm. It's gotten to the point where it hurts to be warmed up that fast and her ears ache and her toes feel like they're burning but she no longer feels like she's going to pass out and never wake up.

She doesn't try to fight the ones who take her wand away again.

They will SEND SOMEONE to HELP KAREN if the merfolk care to consider that a protected part of treaty negotiations and NOT ATTACK that person. OKAY?

The merfolk debate it. They're worried the person will attack to get Karen free and then blame the merfolk for breaking the truce.

Can Miranda call in a favor for having fetched someone to stop the previous kidnapping. Can Timothy call one in for having been the one to stop it. Miranda is the other brown human and Timothy's the orange headed one -

The other brown human can come help if she wants and doesn't bring a wand.

"How am I supposed to - does anybody have warming gear that definitely works underwater -"

Someone goes gillyweed diving on holidays the gear's in her Hufflepuff sister's dorm but she runs off to grab it.

"- I am not going to stop you and I think they're being truthful but I am not confident in my read on other species who I can't see when I don't speak their language and I am in particular not confident if the people making promises are equipped to keep them."

"I could overfly the lake and lower the stuff down."

"I like that better."

Will the merfolk tolerate it?

They see no issue with that at all.