No, at this young of an age, the cygnets are usually with or at least in the very immediate area of the parents. If you do not see them in a couple of days, it is safe to assume they did not make it. Predation, injury and illness are usually the cause of cygnets not surviving because their immune system is very fragile
at such a young age and they are not fast or strong enough to escape predators. The Regal Swan