Sounds like damping off plus you may be a little too early in season (too hot) for a fall cool weather crop for your area.
Make sure you're using a sterilized non soil potting mix. Not jiffy-peat cylinders or peat pots or peat pot compressed cubes (unless you made them yourself).
I won't try to start my fall cool weather crops until the end of this month and I'm in 6b at altitude (4200 feet) (and just barely at 6b). My frost date can be 15 September to 15 October (12 years of personal records) and it's looking like 15 October.
I will then coldframe
my stuff and use floating row cover inside the coldframe as needed and cover the coldframe from half an hour before dark to half an hour after sunup as needed. Going from my early date of 15 September add two weeks for every full growzone (so 9b might be frost on end of October to mid November for that zone)
Starting fall crop I aim for indoor start about 8 weeks before frost date. (I have kale, broccoli and cauliflower going).
I am 6b so 6 1/2 months supposedly, and can eke nine or so months with judicious use of cold frames and floating row cover to produce two cold weather crops and one moderate to hot crop a year, and I gamble with 150 day crops every year. (currently a Chayote that is on top the
trellis and with possibly five to six weeks left, might produce. I am looking for blooms... it is this year's experiment. It is normally a 9b or warmer-cut back in the late fall and mulch and it will come back in early spring in 9b and it WILL produce-I am pushing three zones and the season length this year to try.) I am good with assistance to getting a spring cold weather crop (our hot is usually with us 15 May, and with planting cauliflower and broccoli with help on 15 March, have landed store sized heads just before it hotted up).
Your seedlings might want temperatures in the upper 70s to sprout and start, then cool them down some at four leaves but keep them above 50f at night and no more than 80f during day... once in ground or planter or what have you, after the first month they will tolerate the cool, slow down some in growth but
should produce in 60-90 days once planted.
I purchased this book paperback and it has been worth every cent, on how to push a season and produce some crop nearly year around. He is in Maine, and also talked to, toured, similar areas of latitude and daylight in France. He does some stuff very low key and this book is truly worth it to me (I am frugal with where I spend on
books, though I have a lifelong love with books, and have an extensive reference library dating back a good 45 years...)
https://www.amazon.com/Four-Season-Harvest-Organic-Vegetables-Garden/dp/1890132276/ref=sr_1_1?s=books&ie=UTF8&qid=1504593320&sr=1-1&keywords=four+seasons+gardening
This one was worth my dear dollars and cents... and helped me immensely in pushing my season into viability.