Can you clarify what you mean by mondo grass? Are you referring to regular mondo grass, aka monkey grass or liriope? The name mondo grass or money grass is often used to refer to the clumping grasscovers like liriope muscari or liriope spicata, but they get pretty tall compared to an actual lawn grass. Or do you mean the shorter dwarf mondo grass? Either way, I don't think either will work underneath a tree as a ground cover if children play there. They are heavily clumping and it would seem like the kids would be prone to trip over them---more so the regular mondo grass than the dwarf one. While ground covers are a great lawn substitute in shady areas where there's not enough sunshine for grass, they generally are used to cover the ground and hold the soil in place and not as a lawn substitute where they'll be walked on. Instead, people tend to put down paver type walkways or mulch walkways for people to walk on (which I know is not what you want) and then the lawn cover plants are on either side of the paver pathways. We ha neighbors who had liriope muscari in a significant portion of their yard when I was a kid, and because it was tall, thick/dense and clumping and easy to trip over, we kids avoided those areas like the plague. I've seen dwarf mondo grass used planted closely (it spread more slowly than liriope when I planted it in Fort Worth) as a shorter ground cover in shady areas, but we didn't walk on ours so I have no idea if it would hold up to heavy foot traffic. I've never seen either of them used as a dense ground cover in an area where children play, and I'm guessing there is a reason for that.

Thanks, Dawn. I was talking about the actual mondo grass, not monkey grass. In my research, it also said not to use the dwarf. To keep from having mounds as trip hazards, it said to plant 3 inches apart vs. the usually recommended 4-6 inches. But if you have already had experience with this and it was a no-go, I will continue my search. I’m really getting closer to removing the tree. While I really like the tree, removing it would solve all my lawn problems.

Could you do a big circle of good bark mulch with a border to hold it in? I saw several pictures at Howard Garretts talk that he had done that way and it looked really nice

I agree with Kim that it would be a shame to lose the tree, especially since the grandkids like playing in/beneath the tree. Bark mulch is the easy solution to cover the bare ground, and if any kids fall out of the tree, a 4-6" layer of bark mulch would provide a softer landing cushion.

I stumbled on that video when getting shady area ideas. Homeowner uses mondo in a small area for a lawn look. I can't find it now but I did find a video where a guy turned a much larger area into a mondo lawn

https://youtu.be/0KQxeeUUirA