> It is better for long term growth if kids are humbled at an earlier age
You don't humble a kid. That's not an action any person needs to take, ever. Instead, you expose them to the wonderful things in the world, and they will learn humility and be inspired and motivated at the same time.
If you "humble" them instead, they may learn humility, at the same time as they learn how shitty some adults are, and you seriously fuck with their motivation and inspiration.
I'm aware this may not be quite what you meant; maybe I'm reading too much into your wording.
But while I agree that false praise is bad ("kids are told they are prodigies"), the solution isn't to cut them down; it's to show them more of what's possible (and of course, replace the false praise with simple recognition of how their hard work is paying off, noticing improvement, etc.).
You want to build a drive for self-improvement in them; not convince them they're already at the pinnacle (as if anyone ever is...), nor convince them that they're currently worthless compared to the best (because it's not a competition) -- just notice how they're getting better at climbing, show how it's making their skills/lives/etc. better, and help them along.
> And holding back criticisms now really does a disservice later in life.
Holding back criticism isn't the problem; their limited understanding of the world is. Once you name the problem, the best solutions look quite different; there's no need to tell them their work is low-to-mediocre; they'll be saying it themselves (but with a drive to improve).
There are also still people who will be nasty to kids with the idea of "toughening them up"; this is crap as well (kids can better deal with shitty behavior if they have a solid idea of what is non shitty behavior, and if they haven't been traumatized into having intense emotional responses in conflict), but I don't have time to get into that in depth here....