Hi Matt, I can't think why you waited so long! (Maybe you didn't have anything in your garage with less than 40Kw available??)
A couple of years ago I built an electric minibike for my 4 yr old grandson. It took him about 10 minutes to get the basic idea of controlling it, and the rest of the day to become quite proficient!
It was made from a 16" kid's BMX salvaged from the scrap metal dealer, a 24V industrial brushed motor on a worm gearbox, two 7Ah gel cells, a thumb throttle and 250W brushed e-bike controller, and a few scraps of stainless steel tube.
To get the seat height low enough and fit the motor / gearbox in I had to replace the original back wheel with a 12" one.
He could ride it for about an hour, covering around 7km on flat ground, and it was not too much hassle for me or his father to carry 2 more fully charged batteries in a backpack for the return journey. It had the torque to climb anything it could get traction on, even from a standstill, and a top speed lower than my maximum running speed (Very important!!). At these speeds, the lack of suspension wasn't too much of an issue, and it taught him good riding techniques on rough ground, climbing kerbs etc.
He rode it on bike trails, urban footpaths, beaches, mountain bike tracks, skate parks etc and we never got a negative comment. We were frequently stopped by people wanting to know more about the bike, and saying they thought it was great. Of course little kids are inherently cute and non-threatening, but the near silence must have been a major factor.
He is now riding a 16" Oset and his younger brother will be on the one I built very soon.
A word to the wise though Matt... when you build something for your 4 yr old grandson one day, triple 3220's might be a bit over the top!!!
