peony's, snap draggons, poppys and weak seedlings in general

Might the be getting "cooked" on the windowsill? Try them on a West or East facing windowsill, rather than South, and perhaps start them off right at the South end of the East/West windowsill - so they get some shade from the "reveal" of the window

Underwater rather than overwater. Judge if the plant needs water by feeling how light the weight of the pot is. When it feels really light stand it in a bowl of water for 5 minutes and then feel how much heavier it is, and then use that to judge when it is getting dry in future. The fact that the surface looks / feels dry is not a good measure of how wet the soil is lower down. A small plant (particular when first put in a larger pot, and especially if it is in a relatively large pot) takes a while to drink the water in the soil. As it gets bigger it will drink it more quickly - and need watering more often. When you are having to water it every day, or maybe twice a day, is a good indication that it needs a bigger pot! But whilst it is a small plant in a large-ish pot then too much water means it will take it days to drink it, and in the meantime the water-logged soil will have no air, and the plant can then succumb to rot etc.