This has been a fabulous year for me in raising Monarchs. With the help of a friend I began with 10 eggs and/or caterpillars. While bringing in milkweed leaves to feed those, I discovered about 5 more eggs/caterpillars, so distributed all but one to friends to share the delight.
Many began with their "critter" on a leaf on top of a moist paper towel in a glass container with no lid or a lid of fine netting. I normally place the caterpillars on milkweed leaves inserted into water through the lid of the container. Small margarine tubs worked extrememly well. I normally don't worry about them crawling away until they reach the size of 1/2 my pinkie because they are too interested in eating and remain on the leaves. Then I place the container with the leaves and caterpillar inside a cardboard (office paper) box with the lid off and covered with netting with holes too small for them to escape.
Out of the 15 starts, 5 are now in the chrysalis stage and the "owners" are eagerly anticipating their emergence. Their delight adds immensely to my joy in the process.
I never heard of keeping eggs/caterpillars in plastic baggies. My friend normally places his in glass jars with lids & he has a lot of loss to fungus.
Good luck to the rest of butterfly buddies.