I can't help but be curious of the background of this little trip. Who planned it? How many children are there, and of what ages? How old are your parents?
I'm going to play the Devil's advocate here. I think it's a bit presumptuous to expect your parents to "help out" with your small children when they visit. Grandparenting is the time when they get to enjoy the fun part of being around kids...and let the parents do the work.
Since both your kids are little (not sure about your sister's), pretty much everything to do with them is tough for an older adult. Walking for long periods is tough on the feet, knees and hips, going on rides can be painful on aching joints, and being surrounded by small, noisy children and their oblivious-to-anyone-else parents is tiresome to say the least. They likely do not have the stamina for this, or perhaps the patience.
If you wanted them to spend time with your children, aside from using them as a sitter-service, you should have let them know. Grandparents are not at-will babysitters to be used at your convenience. If you need a sitter, and want to spend time with your parents, hire one and invite your parents out.
I am aware that this sounds a little blunt. For that, I'm sorry. I have a slightly different viewpoint on the grandparent/parent/child relationship: My mother had me at age 16, so I saw how things were when a grandparent had extra expectations put upon them.
I am also in the military. While I was deployed, my mom took care of my two boys, ages 6 and 7 at the time. For 13 months. How's THAT for "helping out?"
When I came back, I vowed to never again force her into a situation where she had to do MY job as a parent. She deserves to be Nana, because she's done her job raising myself and my brother. I am wrong to expect anything else, and have told her so. Strangely enough, letting her know that has made her want to spend more time with the boys than before, because she knows just how much we love and appreciate her.
So, the bottom line is this: Talk to your parents and let them know that it's important to you that your kids have a positive and loving relationship with their grandparents. BUT, when you do, put away your expectations that they should behave a certain way, help you with the kids, or spend that time according to your guidelines, and ask them how THEY would like to grandparent.
Best of luck,
C.