Secreted through their skin, yup! Not in high enough doses to mess with humans without a lot of handling or preparation, but someone Arietty's size?

Now I'm picturing Arietty going into the tank trying to grab that little treasure box or whatever Karla's got stashed in there, and BAM!

Karla comes back twenty minutes later to find a Borrower tripping balls in her frog tank.

Good times.

Also, since they're special frogs, they eat jello and crickets. So don't be surprised if that's in there, too!