Children should be introduced to a whole range of inspiring people. There are some who demonstrate the overcoming of prejudice and disadvantage - we're much better at teaching about this than we used to be. But there are also those who, as George Eliot magnificently put it, "lived faithfully a hidden life and rest in unvisited tombs". Your great-grandmother's story deserves to be told. She may be a powerful moral and practical exemplar, even if her story can only be pieced together from anecdote, oral tradition and conjecture.
It's unhelpful to impose modern views on to the complex flow of history. It doesn't help understanding to present Socrates as a groovy Greek or Shakespeare as a texting Tudor. Explaining that people in the past were similar to us is necessary; but equally important is to summon up that bewitching sense that they were truly alive and yet different from us.
Drama is a good way into biography. Once you've been a Year 5 Michael Faraday or an infant Mary Seacole, your growing sense of historical insight is implanted in body as well as mind. A half-hour spent in another's skin can help make you a free traveller in many centuries.