[Symbiote]

> the main purpose ...

That's absolutely the purpose when you're doing the activity with the child, but there's a decision in the toy shop when you have to choose between Lego and the talking movie-tie-in doll, or between the book and the DVD.

> professional parents

Psychologists or psychiatrists study child development. That's some distance from a "parenting coach", but the coaches probably read the books or study what the psychologists write.

A friend teaches "child development" to 14-16 year olds, in the end they take this [1] exam. It's fairly basic, it mostly deals with babies and infants, but it seems completely reasonable that there's and evidence-based way of dealing with "daddy, there's a monster under my bed".

[1] http://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2... (PDF linked from http://www.aqa.org.uk/subjects/home-economics/gcse/home-econ..., which also has the official answers).