Hi guys, I need help keeping my succulent & cacti seedlings alive!

Many of my seedlings have germinated but the problem I'm having at the moment is keeping them alive. The below photos were taken about a week ago.

Picture 1, both seedlings were doing really well but the one on the left started thinning out 2 days ago at the bottom and yesterday when I went to check on it it looked fried :O. I've kept it on a heat mat and I noticed that the sand dried out a lot quicker as I forgot to turn it off during the day...The one on the right is growing its first pair of true leaves and is doing good so far. These seedlings are about 3-4 months old. I also used a small toothpick to wriggle the right seedling but it is extremely loose, is this also normal? I would have expected a seedling this size to already settle in a little and not be so loose.

Picture 2, just some other photos of some seedlings planted early - mid July. I only placed 2 - 4 seeds per pot so that they have room to grow (if they survive :( ...) and so that I won't have to re-pot until much later.

I need help to determine or get some advice on how often I should be watering the seedlings. Every 2-3 days I get a large container and fill it up with water then dunk the pots in the container so it gets water from the bottom. When I see that the top layer of sand is becoming moist I take it out and put the trays back into my small greenhouse shelf. Is this ok??

This is my first time germinating from seeds so the more advice the better!

I will post up some photos of my dead seedlings later in the day so everyone can see.