@urbanlife it's been a while but I'll dust off my Criminal Law 101 for you to explain why the other girl hasn't committed a crime.
Most crimes require an actus rea (criminal act) and a mens rea (a criminal mind). The other family didn't know they had someone else's boots (If they had them at all, that hasn't been established) therefore lack the intention or knowledge of wrong doing that constitutes part of a crime. If she honestly believed they were her boots, then there is no wrongdoing for her to wear, loose, throw them in the bin, set them on fire etc.
There is also nothing that shows that the other girl even took a pair of boots home from camp. The family might not have realised she'd even lost them for a month and only discovered the lone boot when they went looking for their own.
Unfortunately for OP - you don't get to keep things that aren't yours just because the owner didn't notice you took it for a month.
OP's daughter doesn't get a gold star for 'taking care' of boots she either purposely nicked or carelessly took from her classmate.
We don't know what happened to OP's daughters shoes. We do know what happened to the other girls shoes.
OP should return them immediately if she doesn't want a reputation at school.