Forgive me for not recalling exactly, but when did the grandparents purchase this property? Somewhere in this long thread is a link to a Zillow page showing when it was last bought and sold and if memory serves, they've only owned it for a year or two or three. I don't think it's property that they are apt to know like the back of their hand because they grew up playing in those woods themselves.
That said, if you own it and are an adult, I think it's perfectly reasonable to assume that you'll feel comfortable hiking around and exploring your own property in the middle of a nice day. Kids love adventures and grandparents love to take children on adventures. My grandmothers both lived on farms and both took me out on long walks into the pasture and to creeks and such many times. It's just part of grandparenting and life in the country.
We have no information to know if she did this regularly, if the kids begged to go on a walk or if they were getting restless and she decided they'd all be happier outside than in... we just don't know what led up to the walk in the woods. I can't ascribe a lack of judgment to her unless there is good reason to do so.