The phrase 'hop on the triangle stones only' is almost correct but can be made clearer and grammatically better.
To give a clear instruction to a 5-year-old, you can say:
- 'Hop only on the triangle stones.'
- 'Only hop on the triangle stones.'
These versions make it clear that the child should only hop on the stones that are shaped like triangles, and nowhere else.
So, a good corrected sentence is: Hop only on the triangle stones.