And is it possible that you could find a security guard or a clerk and ask them to watch the child? Most stores now have security. It isn’t a perfect solution, but it’s worlds better than asking a stranger, no matter how “frum” looking. There are plenty of people wearing the “uniform” who are ganz crazy.
As for never having heard of a frum yid killing before, what’s true is that we’ve never heard of a frum yid killing a frum child before. That doesn’t mean that some predator hasn’t killed outside our community and not been caught. The world has never been a very safe place. We’ve been fortunate enough to live in a comparatively well-ordered society in the US, but it isn’t perfect, and sooner or later incidents occur. Hoever, thinking and planning ahead will keep us safer than NOT thinking and planning ahead.