Food/water - You could add a couple of boxes of a drink and a couple of energy bars to make up a section. Try not to make them too attractive to avoid premature use. Cost $3 or $4.

First aid - Half-dozen Band-Aides, a roll of 1" gauze and couple of 2x2 gauze pads make for a first-aid section. Cost $2 to $3.

Shelter and protection - A space blanket and/or a disposable rain poncho. $2 to $4.

Illumination - A cheap but effective keychain light and/or a chem-light. One 2/1.50 party size chem light might serve. $3 to $6.

A 5" by 11" zippered pouch, commonly sold as money and pencil bags, in a bright color, makes a workable storage container. Cost $2 to $5.

Total cost $12 to $22 for a really well rounded kit. You might get by with much less.

Seeing as that any kit you build will end up in school, the mall and church I would avoid any medications, sharp objects, toxic chemicals and anything dealing with fire.

Given the age I wouldn't put too much money and effort into any kit. Girls, because they are usually much more mature and reliable, might be trusted with more stuff. Boys, less. Expect and be prepared to to see the kit dropped into mud puddles, left on the bus, and traded for a cupcake at lunch. Buy supplies from internet discounters and look at making up a half-dozen kits at a time.