Shoes can symbolize wealth because not everyone can afford shoes. Auggie frequently mentions shoes. This means he spends a lot of time looking down because he does not want people to see his face.
Halloween is a symbol of evil and death. August mentions how much he loves Halloween multiple times. I think he likes Halloween so much because he gets to wear a mask and no one judges him or stares at him because they don't know who he is.
Lamb to the slaughter means to do the unknown when their is a chance that their could be a bad outcome. In the chapter "Driving" Auggie's dad mentions the phrase like a lamb to the slaughter when they were talking about sending August to school. This quote mean that Auggie's dad is thinking about sending August to school will be bad because he will get bullied.