We've got a ~4500sq/ft back yard that's probably 90% weeds. Is there an organic/natural way to kill off all the weeds so we can start from scratch?

With kids and dogs and a swimming pool right beside the area, I'd really rather not spray a ton of roundup (or something comparable).

I had one guy suggestion Iron X (http://www.gardensalive.com/product.asp?pn=3721) but I've read mixed reviews.

I've always read about people putting newspaper or black tarp over areas of weed to kill them, but with this square footage I'm just not sure that's feasible.

Here's a close up of what most of the weeds are (though really there are probably a dozen different types):

{{gwi:119492}}