My best advice is to do your own literature search about childhood myopia and possible treatments to slow or stop the progression of myopia. The best searches I have done have not produced any viable, proven treatments, but lots of theories. Check out pubmed on internet to start your literature search.
Sorry to hear about your children. Generally people do not develop myopia until they are into adulthood. Myopia in young children is an indication of some underlying health condition. I know your opthamologist said there is nothing wrong. I would get a second or third opinion - including one from a natural/alternative practioner. Myopia in very young children is generally caused by inability of the eye muscles to focus for reasons that are not well understood if understood at all. Doctors don't know a lot of things even though they present like they know a lot (so they can charge you big bucks). I'd say avoid eyeglasses that only seem to make myopia worse. Glasses treat the symptoms not the cause and in may ways interfere with the body's natural ability to heal (especially in young children whose capacity to heal is so great).
There is growing evidence that exposure to sunlight helps strengthen the eye muscles. Eye muscles are exercised when the eyes are exposed to the sun (of course exercise caution that children do not stare directly into the sun or strong sources of light).
I am a moderate-severe myope whose myopia was induced when I was 7 (made to wear glasses I didn't need) and now at 44 is suffering the consequences of poor decisions by so called professionals and my under-educated parents (who both had 20/20 vision by the way, and who came from families that had 20/20 vision). I have done research and when pregnant with my children, exposed my belly to the sunlight a lot. Babies in the womb are responsive to sunlight and their eye muscles start exercising while still in the womb. A lot of health professionals say there is no evidence for this, but how can it hurt if you exercise common sense when exposing your belly to the sun (not too long, not under strong midday sunlight etc).
When my boy complains now of having fuzzy vision, I send him outside to play as a way of making sure his eyes "recover" so that permanent changes to the eyes do not occur as the body's natural response to fuzziness. This response of the body is what leads to myopia in the long term. Think about it.
yea. Read studies done elsewhere that children should be made to spend more times outdoors to train their eyes to look far. This may reduce progression of myopia.