I think this really depends on the kid. You will know if your grandbaby can handle the gears. Kids do learn very fast. My 7 year old is still on a Trek single speed and can't wait to get big enough to get a bike like dads. Dad can't either actually.

I think if the child is really into biking go for it, but if they just need a bike to abuse then go single speed, kids are really good on them. think about it, I was like 13 before I was able to move on from a single speed, I think most of were in that same age for the conversion? We turned out fine.....I think?