Consult your local animal control officers about removing the possums from under the shed. They may or may not do so, depending if they think the animals pose an immediate threat or not. If they don't remove them, call a privately owned animal control service that will humanely remove the possums and relocate them to another area. Fees for this service vary.
Stand on one side of the shed and look under the floor to see if you can locate the hiding animals. Make sure the animals have a clear escape route directly opposite from where you're standing. Light a few harmless non-explosive smoke bombs and toss them under the shed once they begin to smoke; this will frighten the possums into leaving by making their den difficult to breathe and see in. Once they've left, enclose the crawlspace under the shed to prevent them from coming back.
Look under the shed to see if the possums are there. Usually the babies travel with their mothers, a trait inherent in marsupial species. If the den is vacant, lay "tubes" of panty hose stuffed with dog hair along the perimeter of the crawlspace, which creates the scent of a dog in the immediate area. This will discourage the possums from returning. Simply stuff discarded hose with tufts of hair brushed from a dog and tie the hose at the top to create the tubes.