May one swing on swings on a lawn:
1) If there’s is grass underneath (kotzair issues)
2) even if grass has worn away, can there still be issues of charisha?
I assume that the instance you e asking about is if there is grass in the place where the persons feet swing back and forth, that maybe it will pull the grass out. If the grass under the swing is not high then I don’t see any problem with it, because it is not a psik reisha that it will get pulled outat In the unlikely event that the grass is tall, andit will surely pull some of the grass out, then it is controversial.
If the grass was already worn away then practically the ground is already smooth, and you will not be making it any smoother.
M:B 336, 25, Aruch Hashulchan ibid 25, Shmiras Shabbos Khilchoso 26-19 with footnotes, Migilas sefer Shabbos 11-12.