I get your point, and agree with some of it -
Yes, the parents should be doing the "pushing" not the grandparents. The parents should have been the ones deciding whether the child should go in the first place, and now that the grandparents have paid for it, it is their responsibility to follow through. The concern about the child's feeling and concerns comes in in trying to find out why the child doesn't want to go and assuring them that their needs, emotionally and physically, will be taken care of and mom is only a phone call away.
As for pushing our kids to do something they are not comfortable with, as a parent of a 5 and 2 year old, I can see where you are coming from. I was the same as you when my oldest was that age. Now that I have a teenager and two school age kids, I have done more than my fair share of pushing them to do something that they don't want to do, and they have lived to tell the tale, and sometimes have even thanked me! So, respectfully, telling me that my advice is not very useful is in the eye of the beholder. Going to Disney with loving grandparents that the parents have decided to go along with until the final hours is not the worst thing that can happen to a kid, trust me. Just wait till Middle and High school!