It's best not to complicate this matter, so we're going to give you a KISS (to Keep It Super Simple). 

Take Product A versus Product B, both are bicycles.

Product A promises to be the lightest bike in the world, and markets itself as the best, primarily for this reason.

Product B makes no such promises, but offers a high-end product for less, with an attractive weight advantage over 90% of the industry products in it's class/size etc.

So what's the difference? What is Product A not telling you?

Answer: It all boils down to what is missing from the bike:

1. Spokes: There will be fewer spokes used to construct the wheels. Their gauge will also be a thinner wire material. Spokes will be laced radially instead of criss-crossed. (This makes the wheel as light was possible, spokes are shorter)
2. Rims and Frame Tubes: Wall thicknesses will be minimized to save a few grams. Thinner tubes are lighter but weaker. Aluminum is not like steel, it fails without any warning. Paper thin aluminum tubes are dented and broken easily.
3. Drivetrain: "Grip-shift" mechanisms will be used for gear shifting instead of sturdier trigger shifting because grip-shift is lighter. Regrettably, they are inferior to trigger shift and found primarily on low-end bicycles.
4. Tires: Skinnier tires are used- there's less rubber and therefore less weight, but this puts a limit on safe riding terrain. Better stay on paved surfaces.

Product A weighs a few pounds less than Product B and probably costs 20% to 500% more. (not a typo). "So what? Big deal" we say, but read the specs. Product A has a weight limit, and that weight limit is intended for straight line riding on a sidewalk. If that's all our kids are expected to do, no worries. But take Product A on a rough ride that includes hopping curbs, taking drop-offs or jumping ramps. You can guess what happens. Missing materials and missing spokes translates into missing durability. Parts are loosening. Wheels are getting wonky due to loose spokes. Something will inevitably break at the worst possible moment. Hey what happened to safety? 

Did you know? The UCI sets a limit to how light a professional bicycle racer's bike is allowed to be. Presently this limit is about 6,8 kg (15 lbs)

Let's think about this: The average athlete competing in the Tour de France weighs 145 lbs. What does the UCI know that we do not? They believe that when lightness is taken to an extreme, safety is compromised. 

Kids that are aggressive riders, kids whose parents are mountain bikers, kids who are being groomed for BMX or DownHill or Dirt Jumping are going to push Product A beyond it's limits and something's going to give. At worst there will be buyer's remorse and repairs needed. At worst, it could be a trip to the ER.

Back to Product B. It weighs 2-3 pounds more, but it's engineered to perform like the bikes we used when we were kids: Weight is second to geometry, durability and fitness for intended purpose. There is no weight limit. Wheels have all of their spokes and they are properly criss-crossed to optimize strength. It's built like a brick shit-house.

We caution parents about Product A because it's dangerous when the lightest most expensive bike is used for the kind of riding that we used to do back in the 70s and 80s, when we left the house in the morning and didn't return until dark. We built wooden ramps. We never wore a helmet. We drank from the garden hose. 

"The lightest bike ever" is only suitable for the kids who take no chances, who stay on the sidewalk, and never jump off a curb. Product A is great for these kids, but we will never recommend them for the pump track or the MTB park or the kid who likes to take dares.

"Lightest is best" is an incorrect way to make your buying decision. Buy the bike that fits the rider's taste for risk, adventure, and testing limits. Product B has no weight limit and is likely to be the safer and more durable choice.

Please consider your bike recipient's tendencies when picking out their bike.