At the risk of sounding like a neglectful mom, I think you are in the right here. I think many parents are too protective of their kids, and as a result we have a generation of children who can't understand loss, pain, going without, and consequences in general.
You weighed the risks against the benefits - you knew she most likely wouldn't be seriously hurt (and a scrape really isn't a serious injury, as bad as it looks) and in exchange for a little risk she and the 2.5 year old had a BLAST!
As she grows you won't be able to foresee EVERY possible outcome. You will continue to weigh risks and benefits, and make your decisions based on that. There is ALWAYS a risk, you can't keep her in a bubble forever. She will learn that it's ok to try new things, balance, coordination, strengths and limitations.
On a side note...isn't it always the way? I know of a little man who was about the same age. His father placed him on his feet in the driveway, turned to grab something right behind him, and the little guy fell and scraped his nose and forehead. There's just no stopping these things sometimes!