It clearly got them nowhere on justice for the kidnapping.

Yeah, funny how that works.

Wizards are horrible.

Six wizard children kidnapped a merfolk and merfolk responded by seizing someone they knew perfectly well was not them.

Because they had absolutely no other avenues of getting to the ones who did, or getting assurance it wouldn't happen again. And they didn't seize her, they just didn't let her leave once she landed loudly in their territory after they'd requested wizards stay away from it because they couldn't trust wizards because of the kidnapping thing.

They would have let her die. They never even said what they wanted to DO to the original kidnappers.

...what, no, they weren't going to let her die. 

She was really, really close to dying of cold and it wasn't their idea for Miranda to bring her stuff, what if there hadn't been stuff?

They let her use her wand to warm herself up, and if she'd fallen asleep from the cold they'd have gotten her to the humans, obviously. 

If she'd fallen asleep her bubblehead charm would have probably failed, like, Miranda understands they didn't set out specifically to murder Karen but they did not take adequate precautions she could have died. What did they even want to do with the kidnappers.

Make them apologize to their victim and get reassurance they understood why it was bad and weren't going to do it again. Does Miranda not understand that the kidnapped merperson could also have died or does she just think it's okay if it's merpeople that happens to?

She does not think that's okay that's why she ran and got Timothy and then they nearly killed her uninvolved best friend.

If her best friend stays out of the lake in future there won't be any problems, which is more confidence they won't be kidnapped than any merpeople have.

Karen did not deliberately go for a swim.

Well, they didn't know that when she suddenly dropped into the middle of the lake and wizards are probably capable of making sure people do not by random accident suddenly drop into the middle of the lake if they considered this a priority.

Can Miranda have the merpeople who were responsible for the decision to abduct Karen? She promises she just wants to make them apologize and she will look up how to keep merpeople alive above water first, and she definitely won't involve any merpeople who were not affirmatively in favor of that choice.

Wizards lie constantly about everything so no.

"Minor didn't you say they seemed reasonable," she mutters, "last year - are we talking to the wrong ones or something -"

"I don't actually think they're being that unreasonable. They feel the same way about their friend we feel about ours and the Aurors threatened to kill them all and they just have to take our word for it about the people involved being punished."

"I wouldn't kidnap randomly selected merfolk to hold hostage over it!"

"But if one showed up armed in the castle would you be tempted to delay them in returning home."

"No, I would like to minimize how much time I spend around armed merfolk!"

"And I think feuds are categorically stupid but, like, it makes sense to me that if there are kidnappings which they see no evidence we're inclined to prevent, they'd say 'if any more of you people show up in our territory we'll hold you there' and then that when someone showed up in their territory they would hold them there."

"It's a school! Are they aware it's a school! With children in it! Did they not see Timothy stop those six blithering idiots!"

"I don't know. Once things have calmed down someone who isn't in a screaming rage at them can probably make sure everyone has all relevant information."