Just curious, what is your concern on the 7 year old sitting solo? I believe most airlines allow unaccompanied minors from age 5+ though I'm not positive. I'm sure if you prepped your 7 year old to be on their own (and maybe the 5 year old too) and had a backpack with their snack/water bottle/activities or whatever, they might handle it totally okay. I feel like airplanes are a "safe" situation for a kid to be in a different row. They can't wander off, you're likely able to physically see them (or at least the back of their head) and so you know they're safe. We've also always had the nicest people next to my kids when they've had to sit solo, most people I think are happy to help out with ordering their drink from flight attendant, etc.