Small caterpillars? Did you look at the back of the leaves?

The slugs most likely, but maybe corn earworm (although they will be found on the leaf). Okra has few insect pests that would cause that damage.

kimmq is kimmsr

When I see damage like that to young seedlings, my first thought is earwigs or sow bugs or pill bugs. Visit the greenhouse in the wee hours with a flashlight to catch them in action.

I can tell you from experience that all of the above critters love greenhouses.