Since you are already bringing one, I don't think you need another one. When your 3 year old wants out, maybe your 7 year old can take that time to get in the stroller...to freshen up a bit.
I don't think people should flame about why people bring strollers for older kids. EVERY kid is different and you know your child! See, my soon to be 5 year old DS has ADHD and has the tendency to just run off. I also have a DD who will be 18 months when we go in December. I am bringing a DOUBLE when we go in December. He probably won't use it the whole time, but when he's a little out of control or he JUST gets tired, I want to have it there for him! We're driving, so I am going to bring both strollers...the single and the double.