Bottom of tree has been damaged by trampling - to the point where it's possible it would best be removed. Otherwise it would probably enjoy having well aerated soil filled around it, to bring the grade back up to where it would have been in the past. As long as it was helped out with adequate watering during dry periods, to make up for the fact that the surface of the soil was now higher.

Incorporate tree into bed. use a vining type groundcover, or one that spreads by stolons, to cover roots.

use an ivy or some other type of ground cover but don't discourage the 2year old from caring for the environment

Buy a large amount of Match box cars and trucks. Get a Step Two sandbox.

https://www.amazon.com/Step2-7220KR-Naturally-Playful-Sandbox/dp/B00005U8TE

We had to build a sandbox for my boys, but bought that one with the picnic table to match and a small swing set for the grandkids. It was put under the maple tree that was actually in the neighbors yard, but roots and shade in ours.

Everything stored in the sandbox and stayed dry. They poured sand everywhere.

Keep all the landscaping ideas for later after the toddler years. Don't forget lounge chairs for the adults.

Down the street from me is a lovely home with two massive full-grown maple trees. Just lovely, decades old. A family from out of the country (a Soviet State) bought it. I mention their ethnic background simply to illustrate that they had no knowledge of maple trees.

The maples had extensive roots above ground, much more so than the one pictured above. That’s just what maple trees do.

In their quest for a smooth yard, the owners had ALL the visible roots cut, dug, and removed!!! A massive and pricey project. That was last year. Guess what the trees look like now. Dead. So so sad. And it will cost thousands to have them taken down.

The moral of this story: you can’t fight Mother Nature. Well, you can, but you won’t win.

Thank you for your comments. I don’t think wanting to cover roots and/or figured out what to do with this tree in the middle of our yard is “eliminating all interest from [my daughter’s] environment “ that made me giggle. We also don’t have the money or want to remove this, as I described it, beautiful tree. Our backyard is full of other spots for exploration and fantasy and I foster that imagination daily (and our daughter started walking at 9 months sigh very very aware of the tumbling and falls that come with little ones). Someone mentioned damage from trampling? What about trampling of the roots over the next decades? Placement of the tree has the roots being walked and climbed on throughout the day everyday buy toddler and adults. I don’t want to kill it. We’ve named her and everything :-).

Silver section(s) are where bark has been worn away for an extended period, wood beneath has been weathering. And likely rotting farther back inside - the root crown of your tree is sitting up too high and being adversely impacted. If you want to keep it going you need to do something to protect it.

Also if there is enough damage present at some point the tree may even cut loose and blow over, crush whatever happens to be in its path at the time.

Walking on won't hurt the tree at all. (Ask me how I know, lol.) What does kill the tree is what littlebug described. The cutting and grinding out of most of the major roots. Also, the installing of a lot of dirt and mulch volcanoes.

I hove planted liriope, usually know as Monkey grass around here, around another maple in the side yard on a short steep slope. It is very pretty year round with blue flower spikes. I also dropped daffodil bulbs in for the spring. If you do use liriope, there are two kinds. You really want the clumping, even though it grows slower, instead of the running. I have found that the running seems to have smaller leaves.

ETA: Usually I don't even dig to plant liriope. I plant just like daylillies and iris. Drop on top of ground with a little mulch, water, and away it goes.

I don't know exactly where you are, but look into something like Vinca minor or Asiatic jasmine, both capable of rooting as they spread over the ground. Start with as small of plants as possible. One of the needlepoint ivies out of the house plant/florist department could also work.