They're 5-7 years old, so "Survival" is getting found ASAP.
Loud whistle, pocket survival blanket w/blaze orange on one side and reflective foil on the other. Water resistant LED flash light, maybe an "all night" chem light with a safety pin to attach to torso, all in a zip lock baggie.
Probably best to not give fire starting tool to kids that young. They're not capable of dealing with a fire that gets out of control.
All of this, and this:
An emergency strobe that they can turn on, set down, and leave until daylight (these can be found for little money. Let me know if you have trouble finding them)
One or two of the SOS survival bars
Two or three bottles of water (what do you hear about when they find some kid hugging a tree after two days in the woods? "Scared, scratched, & dehydrated, but otherwise okay.")
I'd recommend two of the emergency blankets (Two is one, yadda yadda)
Weather-dependent shell/jacket (A Frog Togg jacket weighs next to nothing and if they're sitting still, which they should be, it will last plenty long. It's also applicable in any season. They will trap heat in winter and shed water in all seasons, preventing hypothermia.)
As with anything: more important than gear is knowledge. Drill into them that sitting down, staying put and waiting for help is the only answer when they're lost. Also reinforce that they sit down just as soon as they realize or even think that they're lost. Then, show them how to use their stuff while they're waiting for you to find them.