I think it's fine. If you think they will ride out into the street, then I'd park the car sideways at the end of the driveway, or draw a chalk line or put up a row of plastic cones as a reminder not to go past a certain point. I think it's AWESOME that your kids are outside and active, and doing unstructured play together. My neighbor has 3 kids spaced the same as yours, and they are outside all the time rather than watching TV or video games. They know how to entertain themselves.
I'd suggest a certain word they can yell if they really need you. For example, "MOM" can mean "I'm hurt" or "Billy's fighting" or "Will you bring us some snacks".
If you're worried about child predators - I wouldn't. Some people will try to tell you that you have to be hyper-vigilant, but I think it's not a realistic risk. Kids get much more benefit from free play than most people realize, and they need a lot of outside time. So stay with it.