Your added perspective on ballet is helpful. My personal interest comes from several sources.
* I had an older sister who did the whole lessons thing, and was quite good at it. And now she's an attorney. Oh well...at least she's a tax attorney.
* I was forever running across female students at UVa who could drop into splits and stand on their toes on day one. Invariably they came from ballet.
* My Uechi instructor in the latter seventies - a big transition period for me - was David Finkelstein. I'd do week-long visits to New York to train with him during school breaks. David was a big fan of the ballet, and of Barishnikov in particular. He had several ballet professionals in his karate class. Of course males aren't subjected to the toe routine.
Anyhow, I think what was recommended to your daughter and friends about waiting until puberty to do toe is reasonable. My sister did it very young. Still, there's a propensity for women in ballet to develop arthritis in that distal big toe joint. Caution is appropriate. In most sports where weight or stress is to be put on joints, conventional wisdom has it that the really stressful work should wait until the growth plates have matured. This means not doing heavy weightlifting, no plyometrics, etc, etc. In baseball, they often preach for kids to avoid throwing curve balls. But then they let them play football and trash their knees at a young age. Sigh Frankly not a lot of good randomized, controlled work has been done, and the epidemiological work is suggestive but not conclusive. Still...caution is in order.
Find your suggesting about the protection of the shin during roundhouse kotekitae to be right on. I've been experimenting with that while doing ashikitae. If I do straight-ankle roundhouse kicks, I can't get my ankle flexor muscles to contract and protect the shin bone like I can when I bend that ankle at 90 degrees and contract the sokusen. Thus I have a choice between bare bone contact or contracted muscle contact. The former seems better when I want to use the shin bone as a striking weapon. But the latter feels better when getting my roundhouse kick blocked hard by a cross block.
The "reverse roundhouse" you say Bob Campbell showed is in a classical Goju form. The taequondo people also use it. But none of those folks do it with a toe. Makes much more sense that way.
I personally recommend folks develop "internal strength" in their toes before starting to strike with it. I have students bear weight on these toes in their respective orientations. You can do a straight-ankle stand by supporting your weight on something and slowly letting it down on your straight-ankle sokusens. When you do it on a hard floor, the surface gets conditioned more than the muscles and tendons (because of the pain). When you do it on a padded floor, the reverse happens. You can also do the same for roundhouse sokusen by getting in a pushup position on your toes. Once you can do that comfortably, then raise one of your legs to add more weight to the other toe. I believe that a lot of controlled work like this should be done before the unpredictable shock work of striking things.
I also stretch the toes. I do a toe bend-back position, and a toe curled-under position. In both cases, I concentrate on the big toe, and do PNF (proprioceptive neuromuscular facilitation) stretching (isometric contraction in the fully stretched position). Toes, like all joints we use, benefit from range-of-motion work.
Alan Dollar's book has a picture of a piece of a tire attached to the bottom of a makiwara. This is to be used for toe strikes. When I visited the Nebraska dojo, I got a chance to see an example of this that was made and brought over by their Japanese master. It has just the right give. A very nice "Uechi" addition to your classic makiwara. The Okinawans are very creative in this way. Examples like these are my response to folks who start preaching Okinawa religion and the 1000-year-old sacred traditions. We shouldn't copy their thoughts, we should admire and copy their thinking!