The best option would be the rubber mulch. It is made from recycled tires, & never needs to be replaced. Just mark the area, put down a landscaping cover (to prevent grass & weed growth), then level out the mulch.
Another option, that I actually use for my kids is the rubber mat squares. These are 18 inch squares that connect together. You can find them in gray or black anywhere exercise equipment is sold. Or, you can find it in toy stores & dept stores sold as "play mats", usually in various colors. This might be a good option for now since you will have 2 very young children. The mats are easy to clean off with a hose, & you don't need to worry about the kids "eating" it.
But, still let them play in the grass & dirt once in a while. It helps build their immune system.