rob the cradle

cat's cradle

Any complicated structure that appears to be without purpose.

cradle robber

A person who marries or becomes romantically involved with someone who is much younger, or who employs or otherwise engages a young person for a purpose inappropriate for his or her age.

cradle snatcher

A person who prefers to date people significantly younger than themselves.

rob Peter to pay Paul

To use resources that legitimately belong to or are needed by one party in order to satisfy a legitimate need of another party, especially within the same organization or group; to solve a problem in a way that makes another problem worse, producing no net gain.