Kids' book with snakes, magic, riddles and map revealing oil
There was this book that I read in school (2000s but it was a school library book so could have been older) that I think was an isekai where a group of kids gets sent on a quest to collect snake venom from magic snakes.
I think the main character was a boy and there was at least another boy and a girl with him but maybe there were also more than that. From what I remember, there were riddles for the different snakes. One of the riddles was something like BIG BIG SEE SEE which was "too big to see" because the snake was giant. At one point, one of the snakes bites one of the kids (the girl) but the snake venom is magic and makes her pretty.
Also, they have a map at one point that is mostly blank but they get this revealing oil and it spills on the map and reveals more of the map.