This seemingly innocuous sentence, which the reader first interprets unequivocally as "she had provided for them quite beautifully", is an early instance of James's masterful technique of creating unease by probing the ambiguities of language. As the novella progresses, we begin to wonder if "done for" did not perhaps imply that the former governess had doomed the children to destruction.
Other words which initially appear to afford a straightforward interpretation also become riddled with uncertainty. "Charmed", which the governess repeatedly uses to describe her delighted adoration of Miles and Flora, carries undertones of bewitchment. Meanwhile, Mrs Grose's assertion that Quint used to "spoil" Miles is laden with suggestions of moral corruption. The culminative effect of this subtle toying with polysemy is to present language as a medium which is itself haunted; where each word is accompanied by a vague dark shadow of ulterior meaning.