I'm a GM who takes her GD to WDW. The memories we've created there are precious to us both. I can not imagine my ex-D-I-L or my elder son ever telling me I couldn't take her. I also have a 20 month old GS. My other D-I-L and I were talking the other day about how old he should be before I take him for his first trip to WDW. I'm thinking four years old and she's thinking six. However, I know that when the time is right he'll be taking a trip to WDW, just me and him to create our own precious memories. My GD (his cousin) has already said she wants to go with us, but I've told her no, that that time will be for just me and Logan, just like our trips were about her and me. There's too much of an age difference for me to take both of my GK together, but I'd sure like to. Your in laws may not do WDW the same way you would, but I guarantee your kids will have their own precious memories of their time at WDW with just Grandma and Grandpa to spoil them. (For example my GD loves Pimeval Whirl at AK. Her dad hates it. Last January I rode it eight times in a row, because my GD wanted to. We also did the Tea Cups repeatedly. At Disney Quest she designed her own roller coaster and we rode them seven times straight. All things her parents would never do. We had ice cream one night at Beaches and Cream for dinner. She loved it, survived it, and still talks about the great times we had. At home I'd never let her have ice cream for dinner, but we did it, just because it felt naughty.) Don't cheat your kids out of those memories. Their GP won't be around forever, give them a chance for some alone time and let your in laws spoil them in their own way. Let your in laws see WDW through a child's eyes.
I might add I took my GD by myself, with no other adult to help and we did fine. I've also taken her for a week to San Antonio (to stay on the Riverwalk, Fiesta Texas, Sea World, SA Children's museum, and SA zoo) and we had a ball. She normally checks in with her parents by cell phone every couple of days, but it's more to reassure them, than because she misses them.
I might add I took my GD by myself, with no other adult to help and we did fine. I've also taken her for a week to San Antonio (to stay on the Riverwalk, Fiesta Texas, Sea World, SA Children's museum, and SA zoo) and we had a ball. She normally checks in with her parents by cell phone every couple of days, but it's more to reassure them, than because she misses them.