Hi there. Every situation is different. My in-laws live about 10 minutes away. They rarely see or watch our son.
My MIL is just not, how to say it, a nurturing person. When she IS at my house she makes more of a fuss over our dog! Grrrrrr.....In a pinch, for a very good reason, I'll ask, and my FIL will watch him--usually at our house. (Funeral, hospital visit--never for stuff like a date night, etc. Even then though, it's a pain because they don;t KNOW him that well--where his stuff is, what to make him to eat...
They certainly could not be considered "too busy" by any means--they do very, very little.
My MIL told my SIL when her first baby was born "My mom didn't watch my kids, don't expect me to watch yours." (I know, lovely sentiment, eh?) The grandmother always worked and most likely could NOT watch them much. My MIL has never worked. I've taken my cue for them from that.
On the other hand, my mom lives about an hour away and she has watched my son so I can work PT since he was born--He's now almost 7 and in school, so only on school holidays, etc. now. She would even stay over at our house Sunday night and Monday night so I could work Mon & Tuesdays. But she would GLADLY watch him every day if she lived closer. We're very lucky. She plays baseball and football with him, takes him places, etc. And she's 72! They are SO close because of the time they've spent together and they have a great bond. My in laws are missing out on that and I say -- their loss.
Your parents are missing out on a lot of their grandchildren's lives and hopefully, they'll realize it before it's too late.
I doubt that your parents will change much but if they are able to handle all three at once, how about planning (much in advance) a weekend trip for them to their house once every other month? Maybe advance planning and notice would make it easier for them to plan? And 2 hours away is just unreasonable to have them watch them for a day or an evening, so I'm sure that's not what you meant...
I would talk to them about it and see if they are receptive to the idea of an occasional weekend overnight.
Also, I wanted to just suggest that you could tap into other resources--neighbors, parents of your kids friends, etc. that you could trade babysitting with. I know this is not your immediate concern in this post, but I know I have several moms of my son's friends that I do this with and it is really a great thing.
It does HURT me, too, to see my in-laws so disconnected with all of their grandchildren, but in their case, I believe their world is just so small and their global view so tiny that they don't even see it the way I do. Good luck to you!