My boy is turning 4 this summer so I figured it was time to get him on a bike. I went to my LBS and looked around, and finally picked out a little green Raleigh. When I hefted the thing, though, I was SHOCKED at how much it weighed: with training wheels, a little over 22 pounds. 22 LBS!!!
That's just 1.5 lbs less than my 26" fully-geared hardtail MTB, and almost 5 lbs MORE than my 700c fixed gear. This is just ********. Seriously, WTF? The kid weighs less than 40 lbs. Put in context, that's like me (160 lbs) trying to crank a 90 lb bike around. The inertia is incredibly discouraging to the little guy, especially with training wheels. And at $140, the thing isn't cheap, either, despite the crap bottom bracket, crank, headset and stem - all of which are the same size as grownup bikes.
Do light bikes exist for kids? If not, I sense an opportunity for a savvy manufacturer.
Our lbs has a really lightweight 12" glide bike (no pedals). ~$100 I think. Yeah the bike-to-kid weight ratio is amazing. My 7 yr old can ride anywhere for just about any amount of time, but then he practically has to fall on his butt lugging the thing onto or off of his school bike rack.
These are about the lightest I have read about, but they are a 14" vs a 12". Still lighter than the 12" you'll find in the US. The weight difference between Xmart and LBS bikes is not that much. You'll get better rims and maybe better components, maybe more adjustability. The frames will be marginally different.
The glide/run/push bikes are great. I'd recommend one of those first, then see if you need a 12" pedal bike. He might not learn to balance and ride until he is big enough for a 16" or maybe even 20".