2000 Honda Civic cracked manifold

Yes, they could have been breathing it in even without smell. If your breathing in the exhaust fumes you can get carbon monoxide poisoning. The crack may have cracked at once, or it may have gotten a small crack and began to expand after every use of the vehicle so there really is no way of telling how long it has been cracked. I am not a doctor so I can not tell you if your family is ok or not even if they feel well. All I can suggest is to have them checked out along with yourself just to be safe and obviously fix the crack as soon as possible.

Yes, it does smell. If the crack was small but large enough for SOME gas to enter cabin, it is possible that you was still breathing in a small amount of the gas without really being able to smell it. So if it were leaking a slighlty bit then more than likely it wouldn't be enough to harm you anyway, nor enough to produce a strong enough smell. Hope this clears things up a bit. Chances are that you are fine and have nothing to worry about. But once again, I am no doctor so I couldn't really tell you about you and your familys health.