Plastic rims and brake pads
Kool Stop Composite Continental pads are designed for use with composite wheels. They are available in different colors to match the wheel. Many BMX riders complain of streaking on the brake surface. I would carefully clean any residue off the braking surface. Failure to do so will likely result in uneven braking. If these pads are unavailable check for BMX "mag" wheel pads or pads for composite wheels.