If you have booked a seat for the baby, you will not be entitled to a use basinette.
However, basinettes are perfect for a 7 month old, and I would definitely use that option. I have flown my 9 month old and it was the smoothest flight for us These should be able to be pre-booked (as long as you have not booked the seat for the baby)
Anyway, traveling with a small baby, even without a basinette, on your laps is often more comfortable than flying with a 2 year old in their own seat so I wouldn't worry.