This discussion has definitely taken an interesting side-street.
LL’ers and PONY players in California definitely pitch from mounds. I think the standard for <13 yo is less than 10"–it’s probably more like 6", if there is an actual standard.
But, the question of whether here is a standard mound height for kids under 13 yo couldn’t possibly be resolved by looking at the various mounds my son used when he was in LL. They ranged from portables to permanent dirt mounds–the dirt mounds were usually in abysmal shape, having been eroded by weather and use, and only sometimes repaired. Basically the dirt mounds for LL in my part of California can most delicately be described as “variable”.
The portables have their issues too–but absolute height is usually not the problem for those. For the cheapest portables–and most leagues are cost-conscious, sometimes to a fault–the big issues are the length and slope of the approach to home plate and the slope of the sides and back. For some of the cheapest (and most portable) of those mounds, taking a small rocker step in the wind-up can be like stepping off of a ledge.
Back to the OP’s original discussion–in my experience, many youth coaches don’t generally make much of an attempt to teach good throwing mechanics to all of their players, all of whom nevertheless do long-toss warmups from flat-ground very routinely. This may lead to the perception by many kids that flat-ground “throwing mechanics” just don’t matter and/or they are not related to pitching mechanics.
On the other hand, pitching coaches do focus on throwing mechanics and they usually use a mound for instruction, which makes 'em “pitching mechanics”.
Personally, I think the distinction between “flat-ground” throwing mechanics and “mound-based” throwing mechanics is greatly over-rated. In most important respects, throwing mechanics from flat-ground and a mound should be the same.
The slope of the mound should increase the velocity of a throw, and increase the biokinetic forces (i.e., stresses) on the thrower’s body. However, I really think the adjustments that a thrower needs to make for maintaining release-point and accuracy between mound and flat-ground are very minor.
Essentially, good flat-ground throwing mechanics can be performed “as is” on a flat plane that is properly sloped (only 1" drop for every foot of distance).