"Mes grands-parents" when you speak of your grandmother and grandfather. Otherwise you will say "Mon grand-père", "ma grand-mère". The same for "mes parents" when you speak about both of them otherwise it's "mon père", "ma mère". "Mes enfants" when you refer to all your children otherwise you'll say "mon fils", "ma fille" etc....

Your French textbook will cover this in the chapter on possessive articles. Usually, this is covered early in a beginning French course. - masculine and feminine nouns - le père/la mère - definite articles - le garçon, les garçons, la fille, les filles - possessive articles - mon garçon, mes garçons, ma fille, mes filles