Smoke detectors usually start chirping in the middle of a weekday night --- whether or not they have a live battery -- when you need your sleep and no stores are open to get a new battery. So unless you have a new battery handy or very good earplugs, you're likely to be up until dawn. Your children, however, are likely not to be awakened. Children ages 5 to 15 seldom wake up when smoke detectors go off, even at full volume.
Why Smoke Detectors Chirp
Smoke detectors chirp to indicate that the battery is low or dead, so you will be alerted that the device needs a new battery. A dead battery is the same as a missing battery, so even a smoke detector with no battery will usually chirp.
Chirps in Battery-Operated Smoke Detectors
Two possibilities exist if your smoke detector has no battery and is chirping. Many battery-only smoke detectors store energy in case the battery goes completely dead before someone replaces it and in case someone removes the battery to prevent the smoke detector from going off or to prevent it from chirping as it starts to die. This is a safety precaution. A battery-only smoke detector with a low or no battery is useless.
Chirps in Hardwired Smoke Detectors
Another possibility is that the smoke detector is hard-wired. The smoke detector gets the power to chip from the household current. Likely the smoke detector is chirping for the same reason a battery-operated smoke detector chirps --- the battery is low or dead. Hard-wired smoke detectors almost always have a backup battery. People may just assume a hard-wired smoke detector does not need a backup battery (though it does, in case the power goes out), or they may take the smoke detector down and not find the backup battery compartment, which is often hidden on the back side of the detector. Review the owner's manual to find out where the backup is, and replace it to stop the chirping.
Other Causes of Chirping
Chirping in smoke detectors can have other causes. The battery could be loose or installed improperly. The smoke alarm cover may have collected dust or dead insects that interfere with its functioning. The alarm did not reset when the battery was changed. Power surges can also cause hard-wired smoke detectors to chirp. Whatever the cause, a chirping smoke detector will not work properly in case of a fire. If you cannot identify and repair the cause of the chirping, replace the smoke detector.