Unless she absolutely doesn't have the skills or takes crazy risks, just go. Don't go on streets you wouldn't ride for fun - I ride on streets on my commute I wouldn't unless I'm on my way someplace. Ride super-defensive, but a 12 year old should absolutely be able to ride on the street safely. When I was 12 I went anywhere, including along the shoulder of the highway. In fact I rode on roads I'd be a little nervous on now, but never even a close call. You probably did as well, or at least some of your friends did.

I rode last summer with my 4 year old (now 5) on a 14" tire bmx bike. Not on main 4 lane streets, but on any street within our neighbourhood. I rode behind him and coached when necessary.

He rides his bike to school every day in the warmer months, about 2.5km each way. My wife rides with him with the 3 year old in a bike trailer, I've ridden with him on the school run a few times. He's incredibly proud to ride his 'wobbly bike' to school and lock it in the bike rack.