> we probably should do some research by introducing programming languages to children
That might be a good data point, but it is hardly sufficient. One of the biggest problems of objectively evaluating languages is that many languages "behave" differently when the project in question is large (large team, many LOC, long expected lifetime) than when it is small. We have good reason to believe that some languages which excel at productivity when working on small projects, are actually detrimental to large ones. How well a language affects productivity of small projects is certainly important (as small projects are important as is fast prototyping), but in terms of overall significant effect on software development in the industry at large, it is large projects that are far more interesting. Sadly, testing a language on large projects is expensive (and makes using children as the test subjects completely impractical), and as a result some languages have gone completely or near-completely untested in this regard even decades after their introduction. Their benefits (or problems) are therefore often as much folklore and religious belief than anything supported by actual evidence (let alone convincing evidence).
This sad fact, however, is what makes arguments over languages so much fun (for some) and endless.
Personally, my belief is that other than some outliers, the actual effect of modern languages (say any of those made in the past thirty years or so) on large project productivity is measurable but far less significant than some would have us believe (to the point where switching languages for this important class of projects is rarely justified[1]). And when the effect is larger, it has more to do with extra-linguistic features (like GC, performance, monitorability) than any language-level abstraction. But, of course, I can't prove it...
[1]: I am not suggesting languages can't make a bigger impact on large-project productivity, only that the ones we currently have don't. Indeed, more than one language has made such significant impact in the past.