Final answer:
The incorrect word in the sentence is 'grandparent's,' which should be 'grandparents'' to correctly indicate possession by both grandparents.
Step-by-step explanation:
The word used incorrectly in the sentence is grandparent's, which should be grandparents' to show possession belonging to both grandparents. When using possessive forms, an apostrophe followed by an 's' is used to show possession for singular nouns (e.g., grandparent's means something belongs to one grandparent), whereas the apostrophe alone after the 's' (e.g., grandparents') is used for plural possessive (something belongs to both or all grandparents).