That's not how physics or lawsuits work.

If they are using smaller blades, those blades are most certainly spinning much faster than the large blades of a riding or push mower. Thus you're just trading mass for speed and the energy exchange can remain largely the same. You can see this for yourself if you drop items (like a handful of sunflower seeds to simulate rocks) through a spinning ceiling fan or a smaller table fan spinning significantly faster; both can easily throw those seeds that get hit by the blades around a large room. This is how the smaller blades on a lawn mower would even be able to do the same work as the larger blades of a mower.

Also many devices, like table saws, chainsaws, and lawn mowers and considered inherently dangerous to operate. Lawsuits over injury as a result of misuse (like letting children or pets into the yard while mowing) usually have to factor in this inherent danger. There are certain safety measures in place, but I guarantee no mower operator manual suggests letting kids or pets play around a mobile set of spinning metal blades. If you fail to follow the basic instructions in an operators manual while operating a dangerous device, you don't have much ground in a lawsuit.