FWIW, my 4 year old had no problems with the pedal I got (basically this one:
). My 2.5 year old daughter can only push it down about halfway. Your other post referenced a build for a 5-year old and I'm sure there would be no problem. I have used two of these and they both worked exactly as spec'd (0.8 - 4.2V output w/5V supply) and mount fairly well in place of the stock pedal, but they have less throw than the larger ones like the first link.
If you end up with good results with the stock pedal mod, I'd be interested in seeing the final setup.
I retained the stock R-1-2 shifter but wired it so that the motors are always in parallel and in "1st" and "reverse", the switch that would normally switch the motors to series instead shunts off part of the voltage coming from the pedal, effectively limiting it to 50-60% maximum (with a pot to adjust that %, of course). With a microcontroller it should be fairly straightforward to have some lower maximum (70-75%) until the boost switch is pushed
F-150 - 24-36V with homebrew 100A variable-speed motor controller, 4x 775 motors (no, it's NOT all-wheel-drive--look HERE
for more details!)
Pink Rocket Princess Mustang - 15mph, 36V, 4x 775 motors, homebrew PWM controller rev 2....
Hurricane - 24V w/50A step-down controller, 775 motors