Congrats on the new grandson!
You didn't say whether you were flying or not but my wife and I flew quite a bit with our kids, including when they were under one. One word of encouragement is that the age of your grandson was the PERFECT age when we traveled because they were old enough to be out of the newborn phase but not old enough to walk yet. He will be curious but not the "what does this button do?" curious yet. (Funny story: we traveled a European airline once with our kids that put their stewardess call button ON THE ARM REST next to the TV controls. Yeah that was fun...)
If flying (and I apologize if you're not), my one major recommendation would be to try to keep the baby awake prior to boarding the plane so that it could sleep during the flight itself. Obviously this will depend on flight times, but we'd even do this when we'd travel to Europe and departures were in the later evening/night. We'd get settled in the seat and give the baby some time to get aquatinted with the surroundings and then my wife would feed the baby during taxi and it would sleep most of the flight. We always took it as a good sign when other passengers would get up after landing to fetch their carry-ons and comment that they had no idea a baby was on the place. I understand every kid is different, but that always worked for us.
I also always encourage people traveling with small kids, whether they are doing Disney or someone else, is to not push the kids to do more than they can do. I've fallen victim to the trap of trying to do too much and the vacation just doesn't feel fun. Disney should be about slowing down and enjoying your time, more so with little ones.
Another nice gesture you could do for your daughter and son in law is give them some free time. My wife and I did Disney a few years ago with my in-laws and they did this for us. We had our evening meal in the France pavilion. After dinner, my in-laws took our kids, which allowed us to explore Epcot on our own. Now, my kids were a little older at the time (2 and 4), so YMMV, but of all the memories I have of that trip, that precious time with my wife is what stands out most to me. It was magical to be in World Showcase at night, with all the lights, and although I love my kids dearly, it was nice to just be the two of us. Given your grandson's age and depending on your comfort level, you could do things such as walk your grandson around World Showcase while your daughter and son go at their own pace. Or you could take him to another part of MK while they strolled through Main Street, things like this. You'd only ever be about 5-10 minutes away if the baby got fussy, then you could meet up again.
It might just be my love language but as someone that has frequently traveled with extended family, I cherish the small moments that I've been able to share with my wife while on vacation. I've always thought that one of the greatest gifts I could give my kids is to love their mother, and with all the stress of raising kids, jobs, and life in general, I'm very appreciative of the people around me (like my in-laws) that have given me the opportunity to invest in my marriage.
Just my $.02 : ) I hope you have a wonderful trip!
