I had planted some red amaranth seeds in these washed and dettol cleaned seed trays with some boiled cocopeat. However the seedlings that have emerged have their cotyledons stuck to the seed coat. I have used drinking water and kept these in shade.
Don't worry too much about the seed coat being attached to the cotyledon. At some point, once the cotyledon has grown enough, it will either fall off on its own, or you will be able to gently pull on the seed coat and it will come off easily. Until then, just be patient.