I feel like I can give you an educated opinion here as my father had me on both dirtbikes and ATVs by about age 8. My worst injury came on my bike.... (a 3rd degree burn to my leg from the pipe of the bike, honestly a freak accident and not really something to worry about in my opinion) I can't say I preferred one or the other... had a ton of fun on both.
If I had to give you an answer though I would say to start him or her on the bike. Easier to learn when you are young and those skills will stick with you forever. I rode my buddies YZ 250 last year after being off a bike for about 8 years and honestly rode at or above his level.... all because the skills from my childhood came though. I'm sure a lot of my sledding skills and mountain biking skills also came through though. Plus, if you start them on the bike they will have absolutely NO problem transitioning to an ATV.
If I had to give you an answer though I would say to start him or her on the bike. Easier to learn when you are young and those skills will stick with you forever. I rode my buddies YZ 250 last year after being off a bike for about 8 years and honestly rode at or above his level.... all because the skills from my childhood came though. I'm sure a lot of my sledding skills and mountain biking skills also came through though. Plus, if you start them on the bike they will have absolutely NO problem transitioning to an ATV.