By the time of the race it's pretty shady, but bring plenty of sunscreen beforehand.
If you want to be down near the rail you really need to be ready to stand for two hours before the race, which is fine if your kid is cool with that, but if it's a hot or rainy day it can get really tedious.
If it's about just being on the rail, and seeing the end of the race live is not that important to your kid, you can go up closer to the turn to find a spot. I stood there for Charismatic's Belmont and it was pretty cool being far enough away from the stands to actually hear their hoofbeats. The noise from the crowd will let you know what happens at the end.
But hey, he's a kid, someone may likely let him sneak up to the front of your section- most attendees are really accommodating about letting smaller kids get up to where they can see. And the screen is huge; you'll be able to see that from almost anywhere.