1 a group of words established by usage as having a meaning not deducible from those of the individual words (e.g., rain cats and dogs, see the light).
• a form of expression natural to a language, person, or group of people : he had a feeling for phrase and idiom.
• the dialect of a people or part of a country.
2 a characteristic mode of expression in music or art : they were both working in a neo-Impressionist idiom.
ORIGIN late 16th cent.: from French idiome, or via late Latin from Greek idiōma ‘private property, peculiar phraseology,’ from idiousthai ‘make one's own,’ from idios ‘own, private.’
So if we use definition one, we are "by definition" making it harder for kids to learn, as they being human creatures, will naturally try to take in new things by mapping it to what they know.
I prefer the 1st bullet "a form of expression natural to a ... person", in this case those strange creatures we call kids.
Where she talked about using language kids could understand (may have the quote wrong here, but "2 of 3" instead of "2 over 3" for 2/3.