"Our granddaughter wants a small animal."
Translation:Notre petite-fille veut un petit animal.
Grand-mère is an example of a compound word, where two different words have been put together to make a new word. So this happened a long time ago when the French language was still in the process of detaching itself from Latin. In particular, French could not quite decide whether to keep the various endings of the Latin word grandis. At the time grant and granz were as popular as grand. The e ending for the feminine form had not been thought of at the time. By the time grand had become the standard masculine adjective and grande the standard feminine adjective, quite a lot of femine compound words had become fixed with grand rather than grande. When people started trying to standardize the French language this was seen as a problem, so such words were written with an apostrophe, grand'mère, grand'tante etc. Obviously, in time, this spelling was contradicting other French spelling rules, so the apostrophe was replaced by the hyphen. Originally the grand in these words was invariable, so you had grand-mères, grand-tantes etc. It was only during the latter half of the twentieth century that it was decided to allow/encourage an s on the grand in the feminine forms as it was already allowed in the masculine forms such as grands-pères. Sorry this is a long answer. Hope it helps.