I'm with Wendy et al. There's unavoidable risk - we all take those every day, and we take them on behalf of our children. Then there's avoidable risk and, afaic, giving your 7 year old a powerful piece of machinery, then allowing her to ride it, in the dark, illegally, on an unlit road is an avoidable risk.
And no (cod) we don't know all the details and mitigating circumstances and we are just speculating but, hell, if I only offered my opinions on things I was an absolute expert on - well, I'd post rather infrequently.
And ds1 has been on a quad bike - he went to a quad biking party (I posted on here about it - I was really worried). But he was 9, it was supervised, it was in the daytime, the quad bikes had limiters on them so they couldn't go over 5-10 mph, and dh went with him with instructions to pull him out if he felt even vaguely unhappy with any aspect of it. I was still glad when he was home safe and sound though.