Exactly the same conundrum here.
I (kind of) like the the idea of building one up, ticks the father & son time box, but it is time consuming and can work out pretty expensive unless you take your time and pick up bargains (and you don’t really let them decide what parts you put on) along the way.
I saw a father/child build at an event which was a Chinese carbon frame with lots of bling (light race) parts, but it was still cheap as it was 2nd hand, now obsolete 26″ bits ;-)
It worked out really well for them – cheap, light and strong. They might have even posted something on STW so worth searching as a few people have shared their builds I believe.