Because of the set up on the plane, you might be able to move the seat by the window as close to the window armrest as you can, then the middle seat right up next to it, the middle seat might be slightly 'off center' (maybe an inch), which will move it into YOUR seat a little bit. So it might be uncomfy for you. I used the edge of the seat as an armrest for myself, and Im a small person so it didnt bother me.
But sense you have 2 adults, instead of having to worry about it, I would try and get 2 and 2, and put a kid by each window. I would rearface the 15month old because it will be a lot more comfortable for him and he might actually sleep! I'd say put the FF child behind the RF child, so if they kick, they are only kicking their little brother and not a random stranger. Person in front wont be able to recline their seat, but in all of the many flights I flew with ds when he was RF on the plane, I've NEVER had anyone complain about this (flight attendant can move them if needed OR give the person in front a choice of being kicked, or not being able to recline, and put the younger child RF behind your older child - and this also depends on weather your older child has long enough legs to kick the person in front, and weather or not they are a kicker!) Either way the your little guy can still rearface and be more comfy.
You can call in advance to try and switch seats but sometimes they tell you just to wait, so usually I just switch at the gate. On United they always put families in the back, and sense my ds falls asleep before we leave the runway, I dont want to sit next to other people's kids! So I usually get myself upgraded to the 'reserve seats' (this is 2-3 rows behind the bulkhead, so upfront but not in an exit row) I fly a ton and my dad flies even more though so I can get away with convincing them to change me to seats I like better!
But if you have carseats, they have put you where they will fit (they have often given me a middle/aisle which doesnt work, so its on them to figure out new seats). So if you tell them your carseats wont fit side by side, call them up, and they will let you know if you can switch seats now or at the gate.
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