How can my villagers make a rope to go in the cracks?

You make rope in the crafting hut. Two vines make a rope. Make a bunch of ropes to have on hand. You'll need some for the scarecrow, too. Whenever a crack opens up and the lemur goes to it to dig, you can drag a villager there. It will say "If only I had a rope". Drag your villager to the crafting hut and pick up a rope. I don't bother doing that unless the lemur is far enough away, as your guy won't make it in time and you lose a rope.