What an awful situation but in any kind of natural setting you really do have to look out for any snakes, siders, bugs, scorpions and the like. I wonder why the snake did not strike the child or the photographer.
It looks like the snake is just following along with the stroll through the brush. Maybe the snake was trying to make sure the two young ones were safe. I know that is one of the things I always did in making sure that my kids were safe. I would lead them through natural areas so that I could find issues before those issues found us.