Sad that this has been turned into a polarised CM-bashing vs CM-defending thread yet again.
Especially so because, these days, there's no way of knowing whether all these CMs are indeed registered childminders. In many areas, it's more likely that they aren't 
Op doesn't even know this one's name!
However.
I won't comment on whether or not this lady was negligent. I'd need more information than the Op can give with a mere snapshot of the incident.
I'm surprised that Op has posted in AIBU: if she didn't want a CM-bashing thread, the Home Childcare board would have been more appropriate.
I don't think you have reason to report her based on what you have said happened. How can you anyway, when you don't know her name, don't know if she's registered and have no idea who to report her to? 
Snapshot incidents are easily misinterpreted and it's best not to judge too hastily. Let me add my experience of one:
I collect from a school with very relaxed security. People chat and let the kids play while they're waiting. For that reason, I'm vigilant and watch mine all the time.
Except on one occasion when the teacher distracted me to tell me there had been an incident with the child I was collecting. I took my eyes off the kids for a minute or so and, when I looked back, one of them had gone.
After alerting staff and frantically searching, I turned my attention to outside the school. There he was, half way up the road in the care of two self-righteous mothers who proceeded to accuse me of negligence, of having too many children, of being too busy talking and not doing my job.
Never mind that they had been so busy talking, they left the gate open and didn't notice he'd tagged on to their group until they'd got to their cars... 
It's easy to make snap judgements. I have no idea whether your judgement in this case is right or wrong. I don't think you have enough information to decide either.