The correct option is C NO CHANGE
When two phrases are compared with each other, they ought to be of the same grammatical structure. The phrase "they were at language" is consistent to the structure of the phrase "children were better at science", where 'they' modifies the noun 'children' and 'better' is omitted because it's used as a comparative between the phrases. No change needs to be made hence option A is correct. Option B is grammatically incorrect (as one can't do a language). Option C is not parallel in structure to that of the phrase "children were better at science". 'That' is a demonstrative that modifies a noun previously mentioned, since it does not modify any noun here but replaces the phrase "they were at", it's grammatically incorrect hence option D is incorrect.