If I were you, I would buy a simple pulley and a horn cleat. You kids will love tying off the the horn cleat and both should be available at your home center for less than 10$ total.

Horn Cleat:

http://www.homedepot.com/webapp/wcs/stor...ocStoreNum=8125

pulley:

http://www.homedepot.com/webapp/wcs/stor...3&langId=-1