@brian_ls I figured this out and wanted to put my solution here in case it's useful to someone.
There are 2 issues with the event string. First is that it is wrapped with an outer {}, and second it is wrapped with another outer field called 'event'. So I had to deal with each issue separately. I created a simple wrapper class:
public class BaseEventWrapper {
BaseEvent event;
public BaseEvent getEvent() {
return event;
}
}
So when I deserialize, I do so into this class. This takes care of the 'event' element. To remove the outermost {}, I set a property in the ObjectMapper, which all together looks like this:
ObjectMapper mapper = new ObjectMapper().registerModule(new JavaTimeModule());
mapper.configure(SerializationFeature.WRAP_ROOT_VALUE, true);
BaseEvent event = mapper.readValue(newEvent, BaseEventWrapper.class).getEvent();
Enjoy 
Brian.