My boys are 5 and a month shy of 4.  We have a large lot (3 1/2 acres)on a very quiet cul-de-sac.  The boys need much more "run around time" than I can really give them, if I'm directly involved all of the time. My friends are horrified, but I've started letting them play outside without me, with a lot of limits in place.  If they want to ride bikes or play in the cul-de-sac, I bring out work I can do on my laptop or some laundry to fold and sit on the front porch.  If they're in the backyard, as long as they stay in the yard, I feel pretty comfortable staying inside and getting stuff done, while checking on them every once in a while.  They're louder than a pack of banshees, so I usually have a pretty good sense of where they are and what they're doing, without them being directly in my line of sight.

I've just reached the conclusion that while I could spend all of my time obsessing about abductions and accidents, it's in my best interest to just get a grip.  They could fall off of the monkey bars just as easily with me sitting on the bench right next to the play set as they can with me prepping dinner.  Anybody willing to walk into my backyard and try to abduct a child would need to be working hard enough to orchestrate that, that I don't think my letting them play outside or not would really be the major preventative factor.  At some point in time, my child IS going to be greater than two feet away from me, ya know?  They have too much energy and get too bored in the house and I'd rather not clean up after it and be mad at them constantly for something they can't really control.