Thanks all for your input. WW never disappoints. The 4.8kg ISLA looks pretty sick (am not in the UK so may not be easily accessible).
Re kids growing fast, well they don't really grow that fast between 4-10, and beyond that there is really no limit. Some grow half a foot in 3 months especially when they are 12-14.
My son is 5.2 years old and just a hair under 4 feet (118cm for the Euro folks). Kids around us up to 10/11 years old are generally sub-5 feet it seems. Also a lot of adults like running these 20/22 inch wheelers in certain parts of the world. So I am not sweating too much over wheel size for now.
OK, here is how I am going w/ my project. I did quite a bit of research and haven't really seen a good light frame option in this size, so I decided to give the existing Giant a shot first.
It looks like this originally.
Got rid of the kickstand, rear derailleur protector cage (what's the point of protecting a Tourney while I have Dura Ace and XTR ders exposed), decided to keep the chainring guard for now coz it does have some utility and it being plastic is not really heavy.
Then I picked up a knockoff Enve bar for like $30, gave it a few good yanks (I figure if at 185lbs all muslce I can't brake it, my 46lb kid should do fine), cut it short to 48cm, and added a UNO-esque short stem. The combo came to 201g in total.
Voila, 240g saved. Not bad.
Then there's the wheels. The bike came w/ 20 inch 406 wheels and knobby tires with solid steel axles so they weighed around 4kg. It so occurs I have an opportunity to pick up some green 22 inch rims for cheap (apparently there is some surplus wheels from the Goodsmile Racing project ran by some computer game and cartoon company).
These aren't that light at 1700g, but they are interesting. AND GREEN.
Plus they were $40, so whatever. Either way they would be a huge upgrade on the original wheels. If really necessary, I can probably find some 1100g tims for 600g savings.
Seatpost is the fun part. I have had a spare GIANT carbon seatpost from the an early TCR Team ONCE edition. I lost the lower cradle so it's been in the parts bin for like what 15 years??? Anyways, I found a cradle this time and my son's XTC happens to use 27.2 seatposts.
So great, now we can complete the GIANT theme and add some Tour de France flavor. I cut it down to 160g, so that's an instant 150g savings.
I decided not to revamp the drivetrain just yet but may be left for a later date. Also left the tall steerer tube for future needs.
The brakes will be replaced w/ long arm versions as the wheels are getting bigger. The long arm brakes are on the way and should save another 120g off the original ones.
In addition I got some 1 1/8 tires coming in and a lighter cassette on the way too. The complete wheelset w/ tires/cassette should come in at 2.7kg so that's a solid kilogram saved.
Also new pedals are on the way. Not really lighter than the original plastic ones but much smoother/ more functional.
So the unfinished product in the photo is at 7.7kg, so when everything is done, it should be around 8.5kg (19lbs).
Going forward, I feel the BB area and of course the steel fork are two likely candidates for upgrades. Then there is the drive train. Wonder if I should put my M980 shifter/derailleur on there at some point.