Little tomatoes sprouts need to be kept dry around their stems.
An easy way to keep the area dry is to use chick gravel, sold in farm stores.
Just a tiny bit will do the job.
We also bake the starter soil in baking bags for a couple hours. This kills off any unwanted critters living in the dirt.
Also, you can sterilize soil in large quantities in a disposable aluminum pan on the grill - no bag required.
Also, it's a good way to use the remaining ashes on a charcoal grill, after the meal cooking is all done.
Do not use the wife's best bakeware,.. especially indoors !