The two-by-three-foot memorial was installed at Applegate Park back in 2010, along with a statue depicting the two boys.
The Missing Children Monument Project fundraised to have the memorial put up and dedicated to child violence and abduction victims.
Now police are investigating who could have defaced the monument.
Last week, Hulu released a documentary about the case, which focuses on the Stayner family.
In 1972, Steven Stayner was abducted in Merced and held captive for seven years.
He managed to escape and rescue Timothy White from their abductors.
Anyone with information is asked to contact Merced police at 209-388-7725.