Tell us more about what the damage looks like:
Roundish/oval holes in the middles of leaves are usually slug or snail damage:
eventually if left unchecked, the leaves will end up looking like lace, with mostly just the veins left.
Semi-circular holes on the edges of leaves are usually some type of caterpillar, like cabbage worms or others:
as you can see here, caterpillars often leave visible poop behind
Little seedlings cut totally off from the roots, just above ground level and left to wilt and die are cutworm damage:
this picture also shows the culprit, but they do their work at night and ordinarily you wouldn't see the worm, just all your seedlings mown down over night.
etc... so tell us what your damaged seedlings look like...