The most common creaking experiences I've had with cranks on BMX bikes is aluminum crank works loose and will strip if not tightened. Chromo cranks will get cracks that creak.
Typically those types are from jumping, 180/360 type stuff where your weight drops while pedals are horizontal.
This is a fairlly recent one for me. Doing some BMX type stuff on a PK Ripper fixed.
This is