According to a recent Giuliani tweet, the Bidens took a photo golfing with another board member of the Ukraine gas company in 2014. At that time, no Republican opened an investigation. In 2015, The New York Times did a story on Hunter Biden's position on the gas company's board. Republicans still didn't open an investigation at that time either. It wasn't until Biden's father began running for president that Republicans claimed there was a scandal.
The reward Giuliani is offering is a Jr. FBI badge. The badge is given to the graduates of the Junior Special Agent program, which is generally done with elementary school children.