Denis, good job on the kit. It is very important to instill in the child that if they become lost, they will not be in trouble when found by and adult; child mentality makes the lost child afraid of that, and they hide to avoid detection.
Some good suggestions here, except for ones to skimp on quality of kit contents. You want a reliable flashlight and a loud whistle. Good choices, I don't think putting two of that type of torch in the kit is excessive, unless you are sure that one will run about 8 to 10 hours.
Staying put (hugging a tree) may make the difference between being found the first night or not being found at all.
A thought about the ribbon. It's a good idea and it has been in use for years. Another thought is mylar ribbon, as used to scare birds from ag fields. Something like this, but there are other brands and sizes:
http://www.birdbusters.com/shop/catalog/Bird_Scare_Flash_Tape_1_2_in_X_300ft-p-13.html It is thinner, so you can carry more in the same space. It is not only brightly colored (this one silver and red), as is the surveying tape, it is passively reflective, so it gives you about twice the use/visibility as the colored tape alone.
Whichever method you use, you may want the kids to personalize the ribbon/tape with their names every few feet or so with permanent pen, then repackage it. It could be an activity for a safety meeting.