Kids should learn the basic colours on orienteering maps (black, brown, green, yellow, blue, pink) and how they represent categories of features:
Note that kids should start by learning what the general colours mean along with a few key symbols (depending on the area you're working in) and don't need to learn everything right away. Brown and contour features can generally be taught later.
Tip for coaches: Kids tend to instinctively think grass (open field) should be mapped with green and not yellow because 'grass is green'.
The following links point to resources that have been collected and are relevant to this skill (?).
A great set of legends with various explanations of map symbols, etc. in pdf form. Contains one page each for ISOM (2000) and ISSOM.
Description of an orientering version of capture the flag exercise at Orienteering.ca.
Description of / instructions for the orienteering game Ogres (very similar to Capture the flag) at Orienteering.ca.
Description of Vampire O at Orienteering.ca. In Vampire O game, punches and punch cards are used and 'vampires' steal punch cards from orienteers (they then become the new vampire).
Description of / instructions for a 'Poker-O' activity at Orienteering.ca with cards at each control that can be traded out. This version of the game has master maps at each control and is a map memory exercise but it can easily be done where each participant gets a map as well.