I chair a garden at an Elementary school, so my thoughts might not be germain to your age group but...

If you can get some of the teachers/scouting groups to do lessons in the garden (Math: measuring growth/ Actually planting, etc.)

Plant to attract Butterflies/Birds/Hummers. This makes the gargen with more movement.

Also, plants with different textures (ie Lamb's Ear...)

It's nice if it can be tied into the curriculum too--that way you can educate them on the multitude of benefits gardening involves. I lead a school garden at an urban high school. One thing that sounds like you are doing right is not making it too large to start out. (like I did). Good luck.
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I'm in the process of renovating an old Shakespeare Garden in front of our middle school. Most of the plants were high maintenance and kids tramping thru have packed the dirt to cement.

If you're planting near a sidewalk - look for plants that are salt tolerant! We also looked for spring and fall bloomers - things that bloom in the summer are a waste - also there were water issues - remember that someone has to water and weed in the summer.

Also in consideration is winter interest - we're relying on grasses and sedum to provide a foil for the boxwoods.

I consulted with the city's horticulturist for a plan and am interviewing professionals for the install b/c of the issues with the soil.

Remember to call your utilities before you dig!

Part of the Shakespeare garden will be a herb section for the Life Skills department - the kids will plant herbs and label them and use the herbs in cooking.

There are a lot of links out there with Shakespeare Garden lists - if you're interested, let me know. Good luck.