Learning not to run near roads, not being out of sight etc ie danger awareness is something that is learnt gradually, from toddler years up to the end of primary school (actually, teenagers aren’t so good at it either). I wandered off when I was 6 and wasn’t found for an hour. You’re right, it’s not T’s responsibility to keep an eye on her - he’s still learning danger awareness himself at his age. It’s the responsibility of all adults caring for young children (inc grandparents) to teach these skills and, while they’re learning, to monitor carefully what the child is doing. Small kids do run off (and 3 year olds especially so, so I’m sure it wasn’t the first time the museum was in lockdown).
I appreciate her mother is bad at these things but A has had several adults caring for her . What I’m trying to say is that she needs teaching - gently - as all kids do, and it’s the role of every adult in her life to do that. And she’s still at the age where they do these things - she’s curious, not naughty. But A shouldn’t be blamed because, for instance, Js mum lost sight of her .....
