I'm enjoying this thread. My 10 year old has outgrown his 24 inch wheels bike. We inherited an old 26 inch Diamondback (size S?) which is just a little too big and the cranks are too long, but it has big knobby tires and front shocks so he loves that it's a real off road machine (monster truck). We mostly ride pavement, but he likes jumping curbs etc. The Diamondback is a tank, which is good so he hasn't killed it, but it's also heavy like a tank! (33? 34? lbs) I do not like loading it on the rack on the back of the car.
I'm an "avid" cyclist (+/- 1,500 miles a year) and I'm trying to encourage him to enjoy riding. I'm also trying to get him to enjoy 'reality' fun more often than 'virtual reality' fun (online). Also he is very mechanical inclined - very interested in hands on, making stuff, seeing how things work.
So we carefully researched and ordered parts on ebay and we're almost done building his next new bike. Carbon fiber "kid size" (14") MTB frame (direct from China), carbon fork (rigid), hydraulic brakes, 26" wheels, several other carbon bits (stem, bars). It will cost less than a new one at the store, but the real value has been that he's engaged in building it. I'm hoping it will be about 20 lbs when finished. We had a couple of mis-cues in ordering, but were able to return or re-sell the parts. The delays were frustrating, but have built the anticipation. I know this path isn't for everyone, but we're enjoying the ride. YMMV.