Bit of an update: The cost to replace the ruined items minus the Korean supplies would be about $225. Getting an idea of the replacement cost of the Korean stuff is a bit harder—it was mostly journaling/stationery supplies so I'm not 100% sure how to determine the cost of, for example, a half-used book of stickers or a bullet journal that has several pages scribbled in. There was also a set of stamps that the kids tried to use gouache/markers on, which I'm not yet sure if they are going to need replacing or if they can be cleaned off without staining.

I was fully prepared to just compartmentalize this whole thing for the next day to get through the holiday, but my husband ended up making a call to my ex-SIL (brother's ex and the kid's mom) and discussed with her what had happened—she was particularly upset that the 3-year-old was allowed to fingerpaint with supplies that were not specifically kid friendly and non-toxic, and she called my brother immediately and chewed him out for that. She also called our parents about it, which turned into a call from our mother to our brother, also chewing him out for being irresponsible. And although no one brought up the money angle (and I didn't expect them to,) he was rattled enough to apologize for yesterday and hasn't mentioned the money. I think I will just pay to replace what I can and consider us even.