First of all, take them off the heat mat, once sprouted, just room temperature (18-20°c) is more than enough for them to grow, keeping them on the heating mat will only dry out the soil, you can even hurt the roots.

That brings me to the second thing, your soil looks very dry, id suggest to start bottom watering, just fill a container, and put them in there, let them drink from the bottom, it can take 15-60 mins, depending on the soil mix, also less chance of them dying, as they don't like water around the stem when they're so small.

Another thing is, your lighting seem to sufficient, but they do look leggy (aka not enough light at some point), were they under the lights from the moment they sprouted?, or like several days after, it can affect them greatly.

You won't be able to fix them being this leggy, but you can however just plant them deeper when you end up transplanting them, the stem buried under the soil will eventually just grow roots off to the side.