Visiting Grandparents

[busker]

Twenty-something years old, but in this moment, in this setting, you are a child. 
The car’s warm and stagnant air holds you, - 'warm and stagnant', like the air inside a house where the heater's on all the time and windows closed. Like grandparents' homes.
tells you there is nothing you can do but sit still and ride along. 

Outside the windows, sunlight drips from between mountains - a fresh metaphor. But on first read, 'drips' suggested early morning, with the dew association. Turns out it's 'closer to the end of day'. 
as time is slowly falling closer and closer to the end of day.  - why the present continuous? 'time slowly falls' would make more sense in lieu of the above line
The dog is asleep between folded seats, candy is sticky between little sister’s teeth- - this reads like a dreamlike transition from the present to the past
pets and colorful sugar brought along to maintain comforting car ride habits.

The distant radio humming, - 'distant' anything is a cliche
the highway steadily rumbling, - 'steadily' anything is a cliche.
these sounds maintain the stupor.

Suddenly, a thought pushes you out of the soothing, familiar charade; - don't think this line is needed. the next two lines do the job by showing rather than telling
one day you won’t be making the drive as a child, 
one day you won’t be making it at all. 

This is an excellent poem, well above the typical standard for 'Basic'.