I think this is only partly about trust. Maybe some people's kids are very sensible but some will go chasing off after a dog, a cat, a balloon or ball, who knows what!
I think all kids are different and we all think we know our kids and what they would or would not do. But why risk it?
What we do not know if what other factors might happen, like if we got rear-ended while moving the car and ended up talking longer, might the kids go looking for mum?
Just so you know if you are worried about the safety of a child you can anonymously report the parent to the NSPCC. This may sound extreme but of course it does not necessarily mean anything would happen but it might mean they may receive a visit that tells them that some behaviour has been spotted, or so I am led to believe.
It sounds obvious but if you call NSPCC and wish to remain anonymous do not leave your name or identifying details. They actually say that on the phone so there is no confusion whether you want to be identified or not. I have not reported anyone but I did call them about a concern I had, about a child, and they put my fears to rest, so calling was actually very helpful.
Personally, I agree with ILostItInTheEarlyNineties re If she does it again, could you pop over to the kids and chat to them til mum comes back? You could then show her you were a bit concerned and she will hopefully rethink her actions. Although if this is a regular occurrence I may just be tempted to report her.
If one of the kids did run into the road, e.g. the little one, and the older one ran after to protect them, one or both could be injured. No amount of 'I never thought it would happen' can un-break a leg or bring back a life if someone is seriously hurt. To me it's just lazy, is it so hard to walk round the corner with two small kids?
I'd never leave a three and five year old unattended in the street, and yes, I was left in my pram in the front garden, I don't know if it was a safer time, it feels safer in the past and there were fewer cars.