They do, because sea monkies are brine shrimp. But they start off really tiny. After you turn off the airation and let it sit for a bit, put a light on one side of the container and let it sit for 15 minutes or so. You'll notice there are a few tiny orange eggs floating around and a few tinier paler orange little things vibrating around in the water. Those are baby brine shrimp.
They do grow bigger, up to like half an inch or something, but it takes a couple weeks.