Unless one can be found that is factory made then a custom made brake pedal that works with bicycle brake cable would need to be fabricated.
Thanks for the reply. As you suggested I fabricated a brake pedal from an old padlock clasp and a bit of 7/8 tube I found and it works great. I'm sure there are neater ways than using the lever but this way when he's a bit bigger I'll just put it back on the handle bars...
Frank B