I don't see any insects myself but I believe you if you say you've seen them. And yes, if they are eating your seeds, they'll probably do damage to the plants as well. I wish I could give you a suggestion as to what to do but not knowing what kind of insects they are makes it a little difficult. I would probably get some Mosquito Bits to add to your water (you need to soak them for about 30 minutes, then strain them out and just use the water for your plants)... this will kill fungus gnats that live in the soil - but I can't say whether that would kill the insects YOU have... but it would be worth a try.
Good luck