We've got woods to the back of our property. My problem is that we also have about 10 huge oaks in our back yard. So growing grass is almost out of the question.
I'd let it remain natural, but the kids need a place to play. Plus, without vegetation, when it rains, it all drains down and floods my neighbor's yard.
Woods are great for kids. Every kid should have access to a little bit of "wilderness" - and be homeschooled so he can properly enjoy it!