It seems that you are doing everthing correctly. The younger cygnets (juveniles) can basically go out on their own after their parents remove the young birds from their care and this may be the case. The young swans may or may not stay together. Are the adults and cygnets pinioned (unable to fly)?
If not, then they may fly from their parental habitat and a new habitat. If they cannot fly, then they will pair up with the other juveniles on the lake. The Regal Swan