I'm on the side since you are already renting a stroller you might as well get the double.
1. In case the 6 year old has a meltdown or gets suddenly tired he has a place to rest and its shaded.
2. If he doesn't need the stroller, your youngest now has more room to spread out and you now have a place to put your stuff, ie backpack, drinks, popcorn etc etc....
If you only had the 6 year old I would say go with trying not to use one and then if you need one you can always get one there.
1. In case the 6 year old has a meltdown or gets suddenly tired he has a place to rest and its shaded.
2. If he doesn't need the stroller, your youngest now has more room to spread out and you now have a place to put your stuff, ie backpack, drinks, popcorn etc etc....
If you only had the 6 year old I would say go with trying not to use one and then if you need one you can always get one there.