What type of LED lights are those? I have 5,000k flourescents, with about 3,224 lumens, and this year, I got some LEDs from Harbor Freight - 4 foot 5,000k, with 5,000 lumens, and I grew some tomato seedlings under 2 of them - a little less than the 4 fluorescent bulbs I use over 2 trays, but the tomatoes did well, without getting at all leggy, and the leaves were somewhat darker, with none of the purple undersides I often get, but I they don't get leggy under the flourescents, either. I don't think that you have to worry about those curly cotyldons - I notice that on some, and I just figured it's a thing with that variety. The seedlings look fine, to me, except that they seem to be reaching for the light
10-12 hours seems low for growing seedlings - some run them 24 hours, I do 16 hours.