Here is another pic of damaged seedlings.

I would guess slugs or possibly earwigs or pill bugs. They all hide in cool, moist places during the day and feed at night. The last two only seem to do significant damage to very small seedlings, while slugs can wipe out whole seedlings. I've had no luck starting cole crops outdoors in containers (or in ground) because they seem to be so tasty at that tender stage. It's much more likely they will survive if they are four to six weeks old when they go outdoors. You don't usually see slug "tracks." You can control them with diatomaceous earth, but that loses its effectiveness when it gets wet, so you must keep applying it. I like to use iron phosphate slug bait like sluggo or escar-go because it stays effective when wet. All of these organic pesticides are generally considered safe for people, pets and beneficial insects.

Thanks very much, Ohio! I do see earwigs and pillbugs around here, so I'm thinking that might be it. I guess I might need to start my seeds indoors.