I don't bother trying to start caladiums in the open. They're too finicky. Caladiums like a soil temp of at least 70 degrees to start. I use one of those black mixing pans for concrete. They run 6-8 inches deep and are long and wide enough to get things growing.
I put in a 2" base layer of perlite mixed with peat moss, lay the bulbs in, cover with another couple of inches of the peat mix and water thoroughly. CAUTION: the pan is HEAVY once all of that peat is wet. Move it off of your potting bench/table before watering. Put it in an area that gets some sun to help with the warming.
The black color helps keep the peat warm. In about 10 days, you will start to see the shoots popping out. Once they're all up, I remove them in clumps and put them up in pots. You could also then plant them out in the open.
I have had greater than 95% success in sprouting. The only draw back is when you go to remove them from the pan, their roots can be somewhat entangled. A gentle hand works well. This is also why I use the perlite/peat mix instead of soil. The soil is too heavy to easily remove the plants.
