I don't think it's really fair to judge based only on the information you gave.
So many questions remain.... is the child in distress? Is it a mild fever where he/she is just feeling poor, or is it a high fever that indicates a more severe illness?
Is the child very close to the grandparents and comfortable enough with being left with them when sick that the parent going somewhere doesn't cause the child undue stress? How far away is the amusement park? Could they be in touch by phone every hour or so, and be home quickly if need be?
Personally, I wouldn't leave a well
child to go to an amusement park... hate them with a passon...so the whole scenario is a moot point for me!