How can I prevent mildew on my concrete play area in the back yard?

I would not paint concrete Horace. Over time and with any moisture it will bubble and peel. It will also come off if you use the power washer. You will have more of a mess than just green moss. Your area must not get much sun. I suspect that this area is shaded by a lot of trees. Sun would help keep things dryer and less apt to grow moss. You probably would not like to trim any trees because you probably have a beautiful shaded area for your gatherings. I would stick to just using the power washer. It's a pretty easy way to clean off the moss!

Scrub your concrete occasionally with a stiff broom, hot water and a cup or two of baking soda. You can add some Dawn if you wish. The baking soda will prevent the moss from coming back for a little while, but since it is organic it will need to be done every month or so during the growing season. Lime will work too, but it will leave a white film unless your really rinse it off well. You might also try adding baking soda to your pressure washer.

Check out wet & forget outdoor. It's really good for preventing the green & black. It is supposed to work for cleaning as well but I usually don't have the patience for it to work so I cleaned with diluted bleach and then used wet & forget as an inhibitor.

It is available at walmart.com, Lowe's, and QVC. Check out the video on QVC to get a good idea of the product.