I thought I would post this picture so that you can see how they eat. I am still getting away with giving them one fresh Common Milkweed leaf per day. They will need more and more milkweed as they continue to increase in size. The black dots on the paper towel are their frass (poop), which is also getting bigger each day. I change that paper towel out each time I replace an old milkweed leaf with a new one.
For now, I have the larvae in an open, plastic, food container, but it is going to be time to move into a container with a lid tomorrow. Monarchs go through 5 "instars"(I will write more about this when I get the chance to develop the "Rearing"pages). During each instar, the larva will go off by itself and be seemingly motionless. It is best to leave the larva alone at this point. It will shed its skin, eat it, then return back to the milkweed to feed. I panicked when I checked the larvae on the milkweed and only found 4. I thought one had wandered away. I did not see the one (pictured below) on the top of the container going through its instar.