Iceland poppies

I've been asked by a friend to grow some Iceland Poppies, with a view to plant out in a garden space later in the year, and then hopefully have self-seeded plants year on year thereafter.
So a few weeks ago I scattered a packet's worth of seeds over some compost filled cells in a tray, and stuck in the propagator. Seeds were sprouting in a little under two weeks. So far so good.
I've taken the cells out and transplanted the seedlings into slightly larger pots.
I've transplanted them in little clumps as the seedlings are finer than human hair and to separate them at this stage would be impossible.
They're currently in my windowsill growing space with seedlings of other plants; it's not overly warm and the poppies are purposefully not in the strongest sunlight at the front of the bench.
I'm keeping the compost moist.
So far I've lost about 50% of them. They are so delicate and fiddly, even watering them causes problems as they sometimes keel over and then that's it for that seedling; they won't right themselves again.
They seem to be very slow growing, too.
Anyone got any tips? They're starting to get on my nerves to be honest.