When I was about 5 my dad used to pack along a battery operated 6x6 offroad toy for me to pull out and use during rest breaks and in camp. You can pick up much better versions today like
these. They run on one AAA battery, aren't too heavy, and climb over almost anything. (They'll flip over if it gets too steep). The one battery will run quite a while. My aunt bought my son the double pack with the plastic rocks, but you can get them in singles. He loves them, and they'll entertain me too for an hour or so.
Walkie talkies are a good idea for when the younger one learns how to use them. For his 5th birthday I bought my son two battery operated ones (run on 3 or 4 AAA I think) for about $11/pair at Wal-Mart. He hasn't had his birthday yet, so I can't comment on how well they work.
They also make pocket travel games like checkers, chess, and backgammon. The newest ones have magnetic pieces.
Flashlights are an awesome idea, the tiny AAA Solitaire or the 2 AA MagLites are very robust, almost indestuctable.
If they hike one area a lot like the Whites, get the older child his own set of maps to highlight. Maybe even a pocket card for identifying animal tracks or birds.
For older kids, maybe a pocket-sized short-wave radio. Reception should be really good up high. A small camera for the kids to take their own photos is nice too. Some cheap digital versions are out now, some as low as $20. No display and I don't know how good the quality is, but might be worth looking into.
There's a plethora of gadgets out there, some of it fairly cheap. Let us know how you make out.
