Since silkworms don't drink water, they get their moisture from the leaves so they must be fresh, not dried-out. Unless you want to change leaves three times daily, you need a covered container to raise the worms in. It should be almost air-tight to prevent leaves from drying out, but have small air holes for ventilation. Try a transparent plastic cake cover with the handle removed to allow air in through the screw-holes. The photos below show one of these covers, which is about 14" across, plenty of room for about 50 fully-grown silkworms. You can buy these covers and trays from Amazon -click on the photos to get the link.
Amazon sells plastic cake covers and disposable party trays, or you can find them in restaurant supply stores. If the cover has a handle, remove it so air can enter through the holes where the handle normally goes. If there are no holes in the cover, make a few, or prop it slightly ajar so allow air to enter. Make sure to empty the leaves and droppings out every day to prevent mold from forming, which will kill your caterpillars.
To continue to the next stage in development, click on newborn caterpillars.