How old is grandchild and what is their temperament? I have a 3 yo and my biggest piece of advice is to keep expectations low and really let them lead. We’ve done big all out family trips, but a 3 yo is a very different beast. We planned lots of rest, pool time, mid day nap breaks. We got a park hopper for added flexibility (MK morning, back to Beach Club for a nap and Epcot evening was amazing! I’ve started a trip report and the link is in my signature if you’re interested!) my 3 yo is super chill, and we had zero meltdowns because we really just did whatever she wanted to do. You wanna walk through the Moana water garden two times in a row? Sure!
With our big family trip we alll went to the same park each day (MK day1, AK day2, etc) and did our own thing except for a handful of FP or dining options that we did all together. It was fun to split up occasionally, but then meet back up later to sit and eat or ride the safari together. My brother would take his kids back to rest while those of us who didn’t have kids yet went rope drop to close and exhausted ourselves in the best way!