It looks like pinworms. I would give them pyrantel once a week for a few months. Keep vasoline on the anus at night to catch anything the comes out.Wash the area with a shower head every morning. Wash their clothes and linens in hot water and vacuume often. These are hard to get rid of because the eggs live in the air and on surfaces. wipe toilets or changing areas down with alcohol after each use. They also hatch in anal folds and retroinfect that's why its good to keep vasoline there.Vicks works better but too strong for babies.