my DD plays outside on her own all the time. i can see her from all the windows.. and can always hear her! i tend to call outt o her out the windows anyways, even just a 'whatcha doin bub?' and she tells me she is doing... whatever it may be.
at our old place, the back yard was perfect, small, but big enough to play in. there was nohere she could climb up, get out, get stuck etc

