I don't know if there's a sign on the trail stating this, but the site I referenced to does indeed say to keep your speed down when on a bike and as far as I know, this is generally well known advice across the population (strava users take note
There's no reference to the girls age that I can see other than that she was little, but surely old enough to keep up a speed high enough that a sudden halt threw her over the bars. I assume all 5 cyclists to be doing the same speed or collisions with each other would have been happening long before this incident and on that basis, I'd say the lead cyclist who the rest are following is the pace setter and the one at fault because he/she set an unsafe speed for the other 'inexperienced' cyclists to follow and was the one who stopped abruptly and not in a controlled and safe manner. No?
Whether the dog was on a lead or not isn't the issue, as even a dog on a lead can get tangled in a cyclists wheels if the cyclist wasn't paying attention to what's in front of him/her. I'd go so far as to say that the whole point of bringing a dog out into the countryside is to let it off it's lead, let it run about and get some exercise. Or do I start putting harnesses on my children when out walking so they don't get in the way of a cyclist who either doesn't care about the image being given to other cyclists or just not in control of a faster moving machine? Maybe us cyclists who wobble all over the road should get off the roads in case an inattentive motorist hits us?