Driveways can be a very dangerous place for children to play. Your children are at risk from people inadvertently entering your driveway while your they play. The best solution is obviously to simply not use this area as a play space. Realizing that this is not always possible you should take precautions to avoid the possibilities. An easy no cost solution is to simply block the end of your driveway with your car, but lets make sure that your children are inside the house while you do this. Every year children are run over by their own parents while backing out of their driveway.
If using your car as a barrier doesn't work, you could purchase several Children At Play signs or a barrier made of netting that will help keep you little ones from entering the street and alert a driver to the fact that children are at play. These barriers come in several different flavors. The first is the Kidcatcher which it the most substantial but more work to install. Two sleeves are placed in the ground (the best way is to use some quick set concrete around them for better support). two posts are inserted into the sleeves and there is a netting that stretches between them. This is the best system as it is strong enough to stop a small child from running into the street.
The second type is The Kid Safe Driveway Guard which can be purchased in retractable or non-retractable styles and in several lengths. The Kid Safe Driveway Guard is more of a visual barrier and does not have as much strength to stop a child from entering an unsafe area.
No matter which system you choose you should realize its limitations and never leave your children alone when they are at play outside.