It's only 2 measurements of a 3D hole, I assumed depth wasn't specified and it was 6 feet long and a yard wide. Traditionally graves are also 6 feet deep but that's not always practical, so if it's less you should put rocks on top to keep animals from digging it up. Since I'm pretty sure the kid isn't going to bother with a coffin, even if OP is taller than 6 feet, their knees and spine are bendable.
I could of course be wrong and it's six feet deep x 3x3. Not sure how a kid is throwing the dirt up and out, but possible. And easy to dump in a body, it'd just crumple down in there. Less suspicious in appearance too. Stick a little tree on top, water it in.
Or it could be measurements for a circle that is 6ft wide and 3ft deep. We don’t really have enough detail cuz they don’t say the shape but it doesn’t have to be square.