I do keep houseplants but I’ve never seen anything like this on any of my houseplants beforeDo you keep any houseplants? I think that some pest that eats on houseplants is eating on your tender little sprouts! Get back to us. Several people here keep houseplants and can recommend a fix. Sorry, I only overwinter annuals, but I imagine insecticidal soap---NOT WATERED DOWN DAWN!!!!--could help. The Dawn will suffocate the leaves, so it isn't recommended.
Whatever it is has already killed 3 of my seedlingsi'd not worry, that's not enough damage to kill them and once they're planted out they'll likely outgrow any minor imperfections like that.
could be flea beetles or several other things but i'm not sure without being there.
you may have to net the plants to keep the cabbage moths off them (the small white butterflies that are often around).