That will also become crooked and warped in appropriate weather. It only needs to be damp enough and around 0 degrees. Due to frost, the mowing edges without concrete support shift in all directions.
We have the stones purely as a bed border. I didn’t set them in concrete and they shift for me. This is partly due to the growth of the grass and partly due to the drive of the petrol engine. Concrete is certainly nicer, but not as quickly changeable. Our garden is still developing as well. The play tower and sandbox were removed last year, and the bed area was enlarged instead. I think it will be enlarged again in the next few years when less space for kicking and playing on the lawn is needed. When it reaches a fairly final state, then I will set it in concrete.
I have simple concrete paving stones as an edge for mowing for years. Directly in the soil. Of course, sometimes one gets pushed up and another shifted. But overall, it holds very well. It is a quick, simple, and cost-effective solution. Especially if it can/should be temporary.