✩.!.
I can see both sides of this - but specifically regarding the Disney trip - I think the 6 year should go... At age 2 what is he going to get out of it. A 6 year old can at least ride more rides and won't be limited to all the kiddy rides. Plus I wouldn't want to worry about a new potty trained 2 yr old. I think you should let Grandma and Grandpa enjoy time at Disay with a 6 year old. JMO
But again - I see both sides of it. I wouldn't like how they are treating unfairly other times and would mention this to them.