Seedling not growing?

Hi, I wanted to see if anyone has any advice for me. I started some packs of seedlings a month or two ago, petunias and dianthus. The dianthus at first were all really green and healthy and the petunias looked as though we were going to lose the lot. However it seems to be a total turnaround. The petunias are now resembling a jungle and the dianthus are starting to flag. I have one little seedling that just seemed to stop growing after in germinated. It has two leaves, is tiny and at ground level still. Just cant seem to grow. I moved it to a new seed tray thinking that it wasn't getting enough light because it was right next to some petunias which were overshadowing it but it hasn't made any difference and its root is tiny too. Is there any way to trigger a growth spurt in it? Also the other dianthus are plagued by some kind of pest that I can't identify. They are too small to get a good look at but are maggot shaped, very fast movers and about the size of a comma. The affected plants are drooping and turning white. I've repotted them all which seemed to work but then a week later the little sods are back and even gone as far as putting cinnamon on them to try and keep the critters away.

If anyone can help me, it would be much appreciated.

First of all alexa can you tell us where the seedling are, indoors, outdoors, have they had any heat from sun, is the soil very wet, have you watered the seedlings from the top of the soil or from under (bottom watering by sitting the tray / pot in a basin of water till the soil is damp ????, what type of soil are you using, is it seed compost, soil from the garden, old soil you are re-using, all this would help get the best info possible from several other folks who come onto the site. A picture would be better still as it will be easier to identify the grubs you are talking about as they sound like they will destroy all your hard work.
There are grubs that eat old dead leaves roots that fall from other plants, there are others thas eat the actual roots that kill off the seedlings, there are others that suck moisture from the baby plants that also kills them off and IF you treat the wrong bugs with the wrong stuff then you wont achieve anything that can help the problem.
Good Luck WeeNel.

Hi Weenel. They are indoors on the window ledge. They get full sun but not that much really because the weather hasn't been that good lately. Soil is damp to the touch but not soggy watered from the top, started out misting them until they got big enough that direct watering didn't knock them over. They are in seed compost. I have tried to get pictures but as soon as you move enough soil around to uncover one it disappears, far too small and fast to get hold of or even photograph. I tried filming too but the resolution wasn't good enough, they are literally the size of a comma on the screen. Most other websites recommended ground cinnamon so I've tried that but doesn't seem to be making a difference. Haven't seen them for a couple of days but the plants aren't recovering. One has recovered since repotting but its like a plague, as soon as you think you are clear, they reappear and the plants go downhill again.

Are the seedlings large enough to handle by the leaf, or are they smaller than that, The reason I ask is that I would replant the seedlings as in 1 plant at a time, into a seed tray with fresh compost, get rid of the old soil or burn it if you can.
If the seedlins have not grown leaves yet, then I would try to get one of the grubs into a glass jar with lid, take it to garden store and ask them what to use to rid the soil of this pest, remember to say the seedlings are growing in the infested soil and want to save them.
I have a feeling these bugs are something like Vine weevils these are the grubs from the dark coloured adult beetle and the eggs hatch into the grubs you are describing, I know there are several things on the market that kills these grubs but not sure IF these products are safe to use with seedlings in the soil, I would ask at the garden store to be safe. The grubs live in roots and any debris in the soil which is probably why the seedlings are not doing too well.
How about lifting a spoonful gently with a grub in the soil to have it identified properly then take it from there.
Wish I could help you further but without knowing the real culprit I really dont wish to give you the wrong advice.
Maybe someone else will come in and help you out better than I can.
Best Regards. WeeNel.