Covering the roots with soil may harm them. Some trees are more sensitive to added soil than others. Oaks are particularly sensitive to changes in soil level. If the roots die, the rest of the tree will soon follow. You might consider converting the area to a garden filled with shade-loving ground covers. That way your children will avoid the area altogether. To plant shallow-rooted ground covers around the roots of a tree, cover the roots with no more than 1 inch of loose soil so that water and air can penetrate. Buy plugs of the types of plants you want to install (look for ones that thrive in dry shade); plugs are smaller and will disturb the surface roots less than larger container-grown plants would. Carefully dig around and between the roots, and find or make small planting pockets where you can work in some compost and peat moss. After the plugs are in, water well and keep watering so the ground constantly stays a little moist, which helps the newcomers get established. Once they are, they will start to spread and fill in the rooted areas, yet still allow the tree's roots to get air and moisture.