The other day at Wal-mart, I saw some SkyDiamond kites for $1. I bought some on impulse. It was one of the best decisions I've made recently.
These kites are the traditional diamond shape, but they really fly. It was trivial to get them up in the air (unlike many of the other kites I've tried.) My almost-four-year-old daughter could get and keep hers up with a little help.
In the interest of full disclosure, these kites have two quirks. First, their tail is really long, like ten feet. Second, when you put the kite together, their is a black piece that can flip around and be backwards, so if your kite isn't wants to fly upside down, flip that piece around.
We went back to Wal-mart yesterday and got some more :)
Here's the kite.