Grandparents are there to help the child get into mischief they haven't thought of yet.

The joke hinges on affectionate inversion. Grandparents are conventionally imagined as wise guardians who pass down values; Perret flips that expectation by casting them as co-conspirators in “mischief.” The humor depends on exaggeration—grandparents not only permit trouble, they invent new kinds of it—yet the underlying sentiment is warm: intergenerational relationships can create a special space for experimentation, play, and harmless transgression. The line also hints at the freedom that comes with age and distance from parental responsibility: grandparents can prioritize delight over discipline, offering children a broader sense of possibility (and a few stories they may not tell their parents).