Well, the police are pretty sharp when it comes to curfew violations. If a child or teen is not on their home lot (or a friend's lot who has given them permission to sleep over) and does not have an adult from the household with them, they will be caught and escorted home 100% of the time. Sometimes they get caught immediately, sometimes it takes a while...but it always happens. I have tested and proven this with my "homeless teen" experiment.
However, while underage sims are no match for them, the police are very "hit or miss" when it comes to burglars. Even with a burglar alarm, the likelihood of the burglar escaping is about 50%. I've even had brave sims beat a burglar down (one time they beat him down twice), only to have the police officer show up and get manhandled by the burglar.