I have plans to build a play fort/tree house in the woods behind the house. That would be the gathering point for the kids if the house had a fire or break-in. It is across the creek and well concealed.Â
EwB

With clear "sight lanes" towards the main house I would presume? 
Yep, the kids could see the house, but it would be really hard for them to be seen. Also, thinking about a zipline as part of the playground escape plan. Of course, with the "normal play" going on, the kids could find it through the woods blindfolded.
EwB
I was thinking of the white towel/pillowcase hung in a window for the "All Clear" sign. or a dark colored towel or shirt would mean "Stay Put. I'll come to You."
That sort of clear sight lanes.
A red gingham table cloth/napkin or cloth would mean "suppers on come and get it".

Show them the trail markers from the Boy Scout Handbook as well. It becomes a grand game of "follow the clues" [to win the ribbon that gets you out of the dishes for one night or some such] and at the same time they are learning woodscraft that most folks will never even see let alone understand.
A zip line sounds great [and fun] as long as it is not attached to the house or in sight of the house.
Which would defeat the
hidden part of the tree house.
IF you use wire for your zip line show them how to make their own "sliders" out of a wire clothes hanger and a tree branch.
My kids are grown and gone and I am apparently missing some of the fun things we used to do.
