Words "прародители" and "праотцы" are used for very far ancestors, from Adam and Eve to those who lived, say, a few centuries ago. They can be real or legendary, but anyway, it is unlikely that anyone among currently alive knew them personally. So, these terms are not appropriate for grandparents, who are separated from you by only 2 generations and are probably still alive, or died not so long time ago.
"Предки" is a general word for ancestors. It fits a bit better than "праотцы" or "прародители", but still looks awkward, if you mean your Grandma and Grandpa.
I agree with Quassnoi, that for your example the best variant is "деды". This word is widely used nowadays, for example, for participants of the World War II (as collective word for both men and women). It sounds paternalistic, yes, but I don't think it can really insult any women. Use it with stress on Е only, not Ы! With stress on Ы, it sounds like a part of militaristic slang, and is unacceptable in common and, furthermore, official use.