Leaving the '-e' indeed applies when a noun is a 'het-woord', but only when 'een' is used (or when there's no article used, but that does not happen very often). So
- Het lichte kind
- Een licht kind
As for your second question, it's 'het meisje' because 'meisje' is a diminutive. All diminutives are 'het' (except for the plural forms), so it's 'het jongetje' too. The augmentative of 'het meisje' is 'de meid'.
Note that a grammatical gender is not the same as a biological gender.