My two youngest are 7 and 4. We took the double stroller in April. I am the first to say that I am frugal. I have a double Mac umbrella stroller (purchase 4 years ago for a trip to Disney and used many times since). I debated renting but it is alot of money daily and I would be left strollerless at the resort, traveling to and waiting at the bus stops.
The 7yr old can walk and usually did but when he is tired, a place to sit is worth a million, especially at parades. It took the edge off by giving him someplace to sit and regroup. I never second guessed my decision when the seat was empty.
When he wasn't in the stroller, something was, a backpack, camera or souveniors and many times dessert packed to go from the
DDP.
We never split up in the parks so that wasn't a problem. The one time DH took DS7 home early, he walked, almost ran to the bus.
Many times, I have parked the stroller for a few hours in a stroller parking area and found it where it had been left. If it was me, I would bring the double stroller. You won't regret pushing one child in a double stroller, you would have regrets ,if you only brought one stroller that day because you thought that you wouldn't need seats for two.
Good Luck !