wow! let us have a bit more respect for the double back on floor. a sound and consistent double back will take upwards of 1 year of training. this is still considered the holy grail of floor ex. is still the most difficult to land properly and still causes the most injuries.
WHile I agree with the overall sentiment, I think this is a bit of an exaggeration. A double back is far from the holy grail of floor exercise.
As long as the skill is trained properly and not done until the kid is ready, it is neither difficult nor dangerous.
NOW, I want to immediately follow this up by repeating: IF THE SKILL IS TRAINED PROPERLY AND NOT DONE UNTIL THE KID IS READY. In order to train a double back, a gymnast must have a lot of power, excellent technique, and most importantly (and often most overlooked-ly by most coaches) the gymnast must have strong aerial awareness and be confident in their own ability to do the skill. It is not enough to simply be physically capable of turning over twice; a gymnast must not only know exactly where they are in the rotation, they must be confident in their ability to know where they are in the rotation. But once all of this is accounted for, the skill is, as I said, neither difficult nor dangerous.
You are completely correct about the amount of force it puts on the body -- while I would allow 10- and 11-year-olds to train the skill into a pit if they're ready, I would not allow kids that age to put the skill on floor, no matter how gifted they may be.
One thing of note when training a double back -- it is not necessary to try to stick the skill right off the bat. I allow and even encourage kids -- whether doing a double-back on floor, a double flyaway off rings, or a tsuk on vault -- to initially roll out of the landing as they are learning the skill. Once they are comfortable with the skill, then they may try to stick it, but I let them determine when they are ready for this.
Allowing them to roll out lets them get comfortable with the rotation and learn to spot the landing without having to commit to the kickout required to stop the rotation and stick. It also decreases the amount of force on the knees and ankles in the landing.