Grandchildren don’t stay young forever, which is good because Pop-pops have only so many horsey rides in them.

The line uses affectionate humor to capture a bittersweet truth about family time and aging. Children’s rapid growth means the physically demanding, playful rituals of early childhood—like giving “horsey rides”—are temporary. The joke pivots on the double limitation: grandchildren won’t remain small enough to want or fit the game, and grandfathers (“Pop-pops”) won’t remain strong enough to provide it indefinitely. Beneath the comedy is an invitation to savor ordinary, embodied moments of care while they’re available, and to accept change as natural: it’s “good” that children grow up, even if it marks the passing of a stage of closeness.