He helped with the yard work that is important, so don't discourage him by complaining about how he did the Job.

He will learn, and the grass grows.

Did you show him how to do it?

I let my grandson mow my yard a couple of months ago when I was sick. I knew he knew how to drive the tractor as he had done it before. I was amaze by his mowing pattern, then realized he was trying to use a lawn tractor like a zero turn. When he was done the yard looked good, even though he used more gas with his mowing pattern.

I certainly won’t discourage him. Like I said I’m happy for the help. And glad he’s at the age to help. I just prefer not to see all the dirt. I did show him but he got a little overzealous :)

It’s grass, it’ll grown back.

Maybe I need to take a picture straight on. I liked the grass tapering to the curb, not recessed back and scalped straight down

Some people like that sharp cut, dirt trench look.

I like what he did. It’s more of a clean look.

”Tapering to the curb?” Show us.

Did he forget to sweep up?

It'll come back...Do you have a proper edger? I've noticed most gardeners in my area do the edges with weed whackers. It's terrible. :)

Looks good to me., like a too short haircut, it grows back. :)

Check YouTube for a video

Looks like an opportune time to spray the weeds on the other side of the border…using a strip of ( cardboard?) to protect the other side as you spray along the driveway

"I liked the grass tapering to the curb"

Commenting because it looks like this is not possible. As is often the case when soil is graded to the top of sidewalks/driveways/curbs, combined with soil creep and thatch build-up over time, the grass actually sits way above the adjacent hard surface. You would have to shovel down so the soil is 1" below the top of curb, then relay sod.

On the left section sketch is what looks like what was done here for string trimming, with the trimmer head horizontal to the surface. What happens is you chew the edge up, get dieback, and the dirt washes over the curb.

On the right section sketch is how you should trim once it overgrows the curb again. Rotate your trimmer head vertical and then turn an additional angle, so you get a clean grass cut and a thin groove next to the curb to retain any washout.

Watch for weeds as they like that dirt area. The grass will grow back if you can keep the weeds away.

I'd just give it some time and let the grass grow back. It will. I do think technique on how the trimmer is held can make a difference. @3onthetree does a great job showing how position of the trimmer head makes a big impact on the result. Is it possible that Junior is a little too short or doesn't have the strength to hold the trimmer vertically? I'm 5'6" and it's a lot harder for me than my 6'3" husband. And when I was a young teen helping my family with yard work, it was even harder (probably due to strength). In some ways I think they build lawn care equipment to scale for fully grown adult American men and don't think about women like me or youngsters who might also be doing the work.

If you really want it to look good again, you should have a landscaper come in with a dump truck and a tractor to remove "many" inches of soil to bring the level back down to the curb. It looks like the soil slopes down sharply toward the curb but still has several inches elevation above. What you have now is not what was there when the house was built. Owners through the years have been "topdressing" the yard for so many years that you are left with this mound of soil.