I let DS (3.5) out the back by himself but only if I am doing the dishes cause I can see him. If he and DD (8) are out there together then I do what I need to do in other parts of the house.

I do it mainly for safety reasons, and DS likes to turn the outside tap on so I need to be available to turn it off!!!

ETA- On weekends, the kids are out there almost all day.