This should work for the first flight. The few times I’ve flown basic (which like most good FT folks I avoid whenever possible) if there’s a connection involved it won’t let you do it at check in for the second (or any other additional legs) just the first.

I will say sometimes it offers paid for seat selection somewhere the week of the flight. I’ve done this when I’ve booked various low-priced long haul transatlantic solo and want to pay a little more for a window seat on the overnight and an aisle on the way back but it varies and it’s possible only middle options will be available then.

All the kids ages listed would be eligible to travel as accompanied minors so if they can put any of y’all together it’s nice, but it’s just that it’s a nice thing and I wouldn’t expect them to force anyone to move for them. I’d prepare for everyone to be separated. Possibly close by but separated and prepare for 7 middle seats near the back of the plane. It may be a less full flight Or they may have available pairs of seats that at least a few of you get put together or you may get super lucky but basically I’d prepare everyone to be on their own.