One piece of symbolism is the watch shop itself, where the family works. It literally "houses time," so to speak, and when it is raided and shut down, it is as if time has stopped, in a sense.
Another fine piece of symbolism is the hidden Bible that ten Boom uses in this story. The Bible is always a powerful symbol in literature, and it normally equates to religion, spirituality, Christianity, or faith and belief.
In this case, the fact that the Bible is always hidden serves as a metaphor for the hidden faith or beliefs that accompany this story. It also provides the sense that the work being done by ten Boom and her followers is of a secretive or subversive nature. While many believers proudly display their Bibles in modern society, such was not a convenience in the World War II Nazi regime era.