Although not really meant for kids this young, I had to bring our Grandson out to see how he'd take it. He wasn't intimidated in the least. At 3 years, 3 months, he took to it eagerly.
Heading out to the singletrack. If you listen close, you can hear him humming the whole time.
Thirsty work!!! Even a little dirt from one of the diggers he took. Never seemed to bother him, just got right back up and took off again.