Shallow root growth like this is normal for Magnolias and a few other trees like Maple, Beech, and Birch, and covering surface roots could risk "suffocating" them due to reduced access to oxygen. If they present a trip hazard or risk injury from lawn mowers (as it appears may already have occurred), we recommend the mulched zone around the tree be extended outwards.

Even if roots weren't appearing at the surface, using a wide area of mulch (or other plantings, like decorative clumping perennials) around the base of trees as they mature reduces root competition with the lawn and eliminates the risk of wounding form lawn equipment. (Plus, lawn doesn't fare well under tree shade, so it would eventually thin-out in this zone anyway as the tree's canopy becomes fuller.) Keep the mulch layer only a few inches thick (no greater than three) to avoid impacting how much moisture and oxygen reaches the roots belowground.

Miri

Thank you very much. That is helpful.

Karen 

You're welcome.