After looking pretty hard at this stuff for a few days, I am actually having similar thoughts as your advice. I have a lot of other more important projects in the queue already that I never have time for and realize I don't have time to add a custom build construction RC right now.
I am still considering something as a kit because I could squeeze that in with my son without it being a long term deal, but I have just not found the right thing.
I am also considering switching gears and just buying a couple ready to run Kyosho Mini-Z buggies or something like that and setting up a track in the basement to satisfy the Christmas present thing for now. I already have eight 4'x8' tables I built in the basement that were supposed to be for a giant digital slot car track that I am never going to have time to finish designing and setting up. I could use those to get a small scale RC track setup pretty quickly. He would enjoy that.
I can have him help me build the CNC router or help with some other projects already in the works instead to get him some build experience. He just won't be as interested in those or be able to do as much on them though. He is too young to do any CAD, CAM, or design work yet. He is just at the age to start using basic hand tools effectively. I really wanted to sit down and build an RC kit with him, because that is perfect for his age. I want him to experience being a builder instead of just another mindless tablet facebook video game drone. One of the most exciting and rewarding things I did as a kid was building my first R/C car from a box of parts. I still remember getting a new Tamiya Frog and building it way back when it was new and the only real RC had to be built from a kit. I remember all the learning I got from battery terminals falling apart, figuring out how to solder, differentials stripping out, dealing with those old piece of crap mechanical speed controls etc
He would be much more interested in an RC kit than my projects and we could just work on it a little here and there as time permits without taking up a bunch of space etc.
Anyway, to make a long story short, a custom build is out for me, but I am still looking at kits of something. Unfortunately it might not be heavy equipment or construction related since there aren't many options.