I'm after something he can "play" with on the beach with his grandkids, both under 4 yr old. I think that would restrict us to something with a single line only?
You thought the one I linked to didn't look durable? Hard to tell from here but will probably end up getting it and report back.
My uncle takes a group of cubs up Hartshead Pike every year with kites they've made themselves from binbags and bamboo. They love it.