Can a Bombardier CRJ derate the takeoff thrust?
Indeed the CRJ can use reduced takeoff thrust setting.


  Flex power is selected by entering an assumed temperature on the PERF MENU page of the FMS CDU. If the FMS is not available, the assumed temperature can be entered on the EICAS menu page using the EICAS control panel.


It's set up using the CDU or the EICAS control panel. That control panel is on the pedestal aft of the thrust levers, between the radio panels. Do note:


  The FLX indications on the EICAS menu page are only displayed if the FMS has failed or is not available.


As for the activation:


  When programmed, the FLX power setting is activated by the FADEC when the thrust levers are in the TOGA detents.


Source: CRJ700 FCOM



Note on terminology:

Derate and assumed temp/flex are not interchangeable. In derate the takeoff calculations are done based on the derated value remaining the highest even when an engine fails. It can be used to lower the Vmcg/a and in turn the V1 in certain situations (related post).