I would just tell him thank you. You did a nice job on the yard, but I would rather that my grass grow a little more the next time before you cut it. Thank again for a nice job. Then look at it like this hey your getting your grass cut for free.
My Son now 11 got his first 4wheeler when he was 3 and his first dirt bike when he was 4. I would say a dirt bike is safer. Because if it wrecks it kicks away from you unless you get stuck to it. A 4wheeler can roll onto you. My son loves to ride. Just my opinion.