if you have decent wind, a flying wings beetle, or a prism nexus would be two nearly indestructible choices for the dual line.
http://www.chicokites.com/store/index.php?act=viewProd&productId=26http://www.chicokites.com/store/index.php?act=viewProd&productId=28for single line a rok would be a great choice. they're impressive in the sky and are pretty stable. maybe one of these?
http://www.chicokites.com/store/index.php?act=viewProd&productId=434i've not personally flown any of those kites but i have seen a few nexus at the park take a severe beating and come back for more. have heard similar stories of the beetle. a couple of the guys at the fly field also have rok's, just not one of those particular variety, but i always admire how well they fly.
hopefully someone with first hand experience will share. i just wanted to get you rolling. it's quite nice of you and i'm hopeful you can convert one or two into flyers.