Your seedlings may be victims of damping off, a fungal disease that causes the young plants to topple at the soil line. Some gardeners sterilize potting soil before using to prevent this disease. I have found fungi in bags of potting soil after I've opened the bag and used a little, then set it aside for later use. These fungi are usually white and stringy. Ferguson explains that when air does get into the bag, harmless fungi can develop.
Actinomycetes is another fungus that is stringy and white.
Composted manure is a great amendment and nutrient booster. But make sure the manure you use is completely composted. Raw manure is high in salts that stunt growth. I suggest that you consider ordering a few cubic yards of a soil mix from a soil yard to build your beds rather than using bagged potting soils. The blends will provide needed nutrients and the correct pH balance for the plants you wish to grow.